drm/simple-kms: Add plane-state helpers

Just like regular plane-state helpers, drivers can use these new
callbacks to create and destroy private plane state.

v2:
	* make duplicate_state interface compatible with
	  struct drm_plane_funcs

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210208115538.6430-2-tzimmermann@suse.de