drm/sysfb: Merge CRTC functions

Merge the CRTC functions of ofdrm and simpledrm. Replace the code
in each driver with the shared helpers. Set up callbacks with
initializer macros.

Ofdrm supports a gamma LUT, while simpledrm does not. So far ofdrm's
LUT size has been hard-coded in the driver CRTC's atomic_check helper.
Now pass the size of the LUT to the sysfb device. Ofdrm's custom
atomic_flush is still required to apply changes to the LUT. Simpledrm
passes a LUT size of 0, which disables the gamma LUT.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250401094056.32904-11-tzimmermann@suse.de