drm/simple-kms: Use drm_atomic_helper_check_crtc_state()

Simple-KMS helpers guarantee that the CRTC and plane enable
flags are synchronized. Implement this with atomic helpers
drm_atomic_helper_check_crtc_state() on the CRTC side, and
drm_atomic_helper_check_plane_state() on the plane side.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220617103226.25617-3-tzimmermann@suse.de