drm/ofdrm: Add CRTC state

Add a dedicated CRTC state to ofdrm to later store information for
palette updates.

v3:
	* rework CRTC state helpers (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221011150712.3928-3-tzimmermann@suse.de