drm/i915/gvt: ensure -ve return value is handled correctly

An earlier fix changed the return type from find_bb_size however the
integer return is being assigned to a unsigned int so the -ve error
check will never be detected. Make bb_size an int to fix this.

Detected by CoverityScan CID#1456886 ("Unsigned compared against 0")

Fixes: 1e3197d ("drm/i915/gvt: Refine error handling for perform_bb_shadow")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/cmd_parser.c

@@ -1650,7 +1650,7 @@ static int perform_bb_shadow(struct parser_exec_state *s)
 	struct intel_shadow_bb_entry *entry_obj;
 	struct intel_vgpu *vgpu = s->vgpu;
 	unsigned long gma = 0;
-	uint32_t bb_size;
+	int bb_size;
 	void *dst = NULL;
 	int ret = 0;