drm/crtc-helper: Add a drm_crtc_helper_atomic_check() helper

Provides a default CRTC state check handler for CRTCs that only have one
primary plane attached.

There are some drivers that duplicate this logic in their helpers, such as
simpledrm and ssd130x. Factor out this common code into a CRTC helper and
make drivers use it.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221011165136.469750-5-javierm@redhat.com