drm/mgag200: Remove special case for G200SE with <2 MiB

Remove old test for 32-bit vs 16-bit colors. Prefer 24-bit color depth
on all devices. 32-bit color depth doesn't exist, it should have always
been 24-bit.

G200SE with less than 2 MiB of video memory have defaulted to 16-bit
color depth, as the original revision of the G200SE had only 1.75 MiB
of video memory. Using 16-bit colors enabled XGA resolution. But we
now already limit these devices to VGA resolutions as the memory-bandwith
test assumes 32-bit pixel size. So drop the special case from color-depth
selection.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601112522.5774-2-tzimmermann@suse.de