drm/ofdrm: Support color management

Support the CRTC's color-management property and implement each model's
palette support.

The OF hardware has different methods of setting the palette. The
respective code has been taken from fbdev's offb and refactored into
per-model device functions. The device functions integrate this
functionality into the overall modesetting.

As palette handling is a CRTC property that depends on the primary
plane's color format, the plane's atomic_check helper now updates the
format field in ofdrm's custom CRTC state. The CRTC's atomic_flush
helper updates the palette for the format as needed.

v4:
	* use cpu_to_be32() (Geert)
v3:
	* lookup CRTC state with drm_atomic_get_new_crtc_state()
	* access HW palette with writeb(), writel(), and readl() (Ben)
	* declare register values as u32

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221011150712.3928-5-tzimmermann@suse.de