librem_15v4: Disable i915 compressed framebuffer

Compressed framebuffer requires the driver to track updates to the
framebuffer from the CPU and update the compressed framebuffer.  This
doesn't work if we kexec into an OS that will use the linear
framebuffer, so disable it.  (The OS kernel can still use compressed
framebuffer if it has i915.)

Linux 5.8 enabled compressed framebuffer on more chipsets using i915,
which is why this stopped working.

memtest86+ and Debian (manually blacklisted i915, comparable to
netinst) now boot correctly on Librem 15v4.  This will need to be
enabled for other boards too.

Signed-off-by: Jonathon Hall <[email protected]>

config/coreboot-librem_15v4.config

@@ -9,5 +9,5 @@ CONFIG_HAVE_ME_BIN=y
 CONFIG_TPM_MEASURED_BOOT=y
 CONFIG_PAYLOAD_LINUX=y
 CONFIG_PAYLOAD_FILE="@BOARD_BUILD_DIR@/bzImage"
-CONFIG_LINUX_COMMAND_LINE="iommu=pt quiet loglevel=2 video=eDP-1:1920x1080 drm_kms_helper.drm_leak_fbdev_smem=1"
+CONFIG_LINUX_COMMAND_LINE="iommu=pt quiet loglevel=2 video=eDP-1:1920x1080 drm_kms_helper.drm_leak_fbdev_smem=1 i915.enable_fbc=0"
 CONFIG_LINUX_INITRD="@BOARD_BUILD_DIR@/initrd.cpio.xz"