drm/mgag200: Force 32 bpp on the console

G200ER does not seem to support 24 bpp, so force the console to
use 32 bpp. The problem got introduced when commit 73f54d5d
("drm/mgag200: Remove special case for G200SE with <2 MiB") changed
the preferred color depth from 32 bit to 24 bit.

A setting of 24 is the correct color depth, but G200ER doesn't seem
to be able to use the respective RGB888 color format. Using 24-bit
color with forced 32 bpp works around the problem.

Reported-by: Wang Yugui <wangyugui@e16-tech.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Wang Yugui <wangyugui@e16-tech.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Fixes: 73f54d5d ("drm/mgag200: Remove special case for G200SE with <2 MiB")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220915150348.31504-1-tzimmermann@suse.de