drm/simpledrm: Use drm_atomic_get_new_plane_state()

Lookup the plane's state in atomic_update with the helper
drm_atomic_get_new_plane_state(). Also rename the helpers'
state arguments. No functional changes.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220922130944.27138-3-tzimmermann@suse.de