drm/vmwgfx: Create debugfs ttm_resource_manager entry only if needed

The driver creates /sys/kernel/debug/dri/0/mob_ttm even when the
corresponding ttm_resource_manager is not allocated.
This leads to a crash when trying to read from this file.

Add a check to create mob_ttm, system_mob_ttm, and gmr_ttm debug file
only when the corresponding ttm_resource_manager is allocated.

crash> bt
PID: 3133409  TASK: ffff8fe4834a5000  CPU: 3    COMMAND: "grep"
 #0 [ffffb954506b3b20] machine_kexec at ffffffffb2a6bec3
 #1 [ffffb954506b3b78] __crash_kexec at ffffffffb2bb598a
 Rust-for-Linux#2 [ffffb954506b3c38] crash_kexec at ffffffffb2bb68c1
 Rust-for-Linux#3 [ffffb954506b3c50] oops_end at ffffffffb2a2a9b1
 Rust-for-Linux#4 [ffffb954506b3c70] no_context at ffffffffb2a7e913
 Rust-for-Linux#5 [ffffb954506b3cc8] __bad_area_nosemaphore at ffffffffb2a7ec8c
 Rust-for-Linux#6 [ffffb954506b3d10] do_page_fault at ffffffffb2a7f887
 Rust-for-Linux#7 [ffffb954506b3d40] page_fault at ffffffffb360116e
    [exception RIP: ttm_resource_manager_debug+0x11]
    RIP: ffffffffc04afd11  RSP: ffffb954506b3df0  RFLAGS: 00010246
    RAX: ffff8fe41a6d1200  RBX: 0000000000000000  RCX: 0000000000000940
    RDX: 0000000000000000  RSI: ffffffffc04b4338  RDI: 0000000000000000
    RBP: ffffb954506b3e08   R8: ffff8fee3ffad000   R9: 0000000000000000
    R10: ffff8fe41a76a000  R11: 0000000000000001  R12: 00000000ffffffff
    R13: 0000000000000001  R14: ffff8fe5bb6f3900  R15: ffff8fe41a6d1200
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 Rust-for-Linux#8 [ffffb954506b3e00] ttm_resource_manager_show at ffffffffc04afde7 [ttm]
 Rust-for-Linux#9 [ffffb954506b3e30] seq_read at ffffffffb2d8f9f3
    RIP: 00007f4c4eda8985  RSP: 00007ffdbba9e9f8  RFLAGS: 00000246
    RAX: ffffffffffffffda  RBX: 000000000037e000  RCX: 00007f4c4eda8985
    RDX: 000000000037e000  RSI: 00007f4c41573000  RDI: 0000000000000003
    RBP: 000000000037e000   R8: 0000000000000000   R9: 000000000037fe30
    R10: 0000000000000000  R11: 0000000000000246  R12: 00007f4c41573000
    R13: 0000000000000003  R14: 00007f4c41572010  R15: 0000000000000003
    ORIG_RAX: 0000000000000000  CS: 0033  SS: 002b

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Fixes: af4a25b ("drm/vmwgfx: Add debugfs entries for various ttm resource managers")
Cc: <stable@vger.kernel.org>
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240312093551.196609-1-jfalempe@redhat.com

drivers/gpu/drm/vmwgfx/vmwgfx_drv.c

@@ -1444,12 +1444,15 @@ static void vmw_debugfs_resource_managers_init(struct vmw_private *vmw)
 					    root, "system_ttm");
 	ttm_resource_manager_create_debugfs(ttm_manager_type(&vmw->bdev, TTM_PL_VRAM),
 					    root, "vram_ttm");
-	ttm_resource_manager_create_debugfs(ttm_manager_type(&vmw->bdev, VMW_PL_GMR),
-					    root, "gmr_ttm");
-	ttm_resource_manager_create_debugfs(ttm_manager_type(&vmw->bdev, VMW_PL_MOB),
-					    root, "mob_ttm");
-	ttm_resource_manager_create_debugfs(ttm_manager_type(&vmw->bdev, VMW_PL_SYSTEM),
-					    root, "system_mob_ttm");
+	if (vmw->has_gmr)
+		ttm_resource_manager_create_debugfs(ttm_manager_type(&vmw->bdev, VMW_PL_GMR),
+						    root, "gmr_ttm");
+	if (vmw->has_mob) {
+		ttm_resource_manager_create_debugfs(ttm_manager_type(&vmw->bdev, VMW_PL_MOB),
+						    root, "mob_ttm");
+		ttm_resource_manager_create_debugfs(ttm_manager_type(&vmw->bdev, VMW_PL_SYSTEM),
+						    root, "system_mob_ttm");
+	}
 }
 
 static int vmwgfx_pm_notifier(struct notifier_block *nb, unsigned long val,