SeaVGABios/cbvga: Fix bpp for coreboot framebuffer

SeaVGABios/cbvga: Fix bpp for coreboot framebuffer

Commit 4b42cc4 [SeaVGABios/cbvga: Advertise correct pixel format] neglected
to wrap the cbfb mask size components in GET_FARVAR(), which resulted in a
bogus value for bpp, breaking output on most/all devices.  Fix this by
adding GET_FARVAR() as appropriate.

Additionally, some newer ChromeOS devices still fail even with this fix,
so fall back to using the coreboot reported bit depth if the calculated
valid is invalid.

TEST: build/boot a variety of devices (google/[reef,eve], purism/librem_skl)
using coreboot framebuffer init, verify SeaBIOS boot menu prompt visible.

Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>