config/coreboot-librem_11.config: intel_iommu=igfx_off on Heads cmdline

intel_iommu=igfx_off is needed on the Heads kernel command line for
memtest86+ to work.  Without this parameter, the screen blanks when
memtest86+ starts testing.

This is unique to Librem 11, probably because it is the only device
using FSP GOP for graphics init in coreboot.  (libgfxinit does not yet
support Jasper Lake.)

Signed-off-by: Jonathon Hall <jonathon.hall@puri.sm>

config/coreboot-librem_11.config

@@ -176,7 +176,7 @@ CONFIG_BOARD_PURISM_BASEBOARD_LIBREM_JSL=y
 CONFIG_FSP_TEMP_RAM_SIZE=0x28000
 CONFIG_HEAP_SIZE=0x8000
 CONFIG_EC_GPE_SCI=0x50
-CONFIG_LINUX_COMMAND_LINE="quiet loglevel=2"
+CONFIG_LINUX_COMMAND_LINE="intel_iommu=igfx_off quiet loglevel=2"
 CONFIG_BOARD_ROMSIZE_KB_16384=y
 # CONFIG_COREBOOT_ROMSIZE_KB_256 is not set
 # CONFIG_COREBOOT_ROMSIZE_KB_512 is not set