drm/mgag200: Store primary plane's color format in CRTC state

Store the primary plane's color format in the CRTC state and use
it for programming the CRTC's gamma LUTs.

Gamma tables (i.e., color management) are provided by the CRTC, but
depend in the primary plane's color format. Store the format in the
CRTC state and use it. This has not been an issue with simple-KMS
helpers, which mix-up plane and CRTC state to some extent. For using
regular atomic helpers, it's necessary to distinguish between the two.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728124103.30159-6-tzimmermann@suse.de