drm/simpledrm: Convert to atomic helpers

Replace the simple-KMS helpers with the regular atomic helpers. The
regular helpers are better architectured and therefore allow for easier
code sharing among drivers. No functional changes.

v3:
	* remove empty CRTC helpers atomic_{enable, disable} (Javier)
	* unconditionally run drm_atomic_helper_check_plane_state()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220801135028.30647-6-tzimmermann@suse.de