drm/ast: Handle primary-plane format setup in atomic_update

Several color registers are programmed in the DPMS code of the CRTC's
atomic_enable helper and the primary plane's atomic_update. It requires
the color format and the display mode.

Both code paths handle different cases: the DPMS's code will not be
executed if the color format changes without a full mode switch. The
plane's code only runs if the color format changes, but ignores
display-mode changes.

The color format is a property of the primary plane, so consolidate all
color-format code in the plane's atomic_update. Remove it from the DPMS
helper.

v2:
- clarify commit message (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240627153638.8765-5-tzimmermann@suse.de