drm/simple-kms: Support custom CRTC state

Simple KMS helpers already support custom state for planes. Extend the
helpers to support custom CRTC state as well. Drivers can set the reset,
duplicate and destroy callbacks for the display pipeline's CRTC state
and inherit from struct drm_crtc_state by embedding an instance.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714142240.21979-12-tzimmermann@suse.de