Implement MIPI DBI planes with struct drm_shadow_plane_state, so that the
respective drivers can use the vmap'ed GEM-buffer memory. Implement state
helpers, the {begin,end}_fb_access helpers and wire up everything.

With this commit, MIPI DBI drivers can access the GEM object's memory
that is provided by shadow-plane state. The actual changes to drivers
are implemented separately.

v2:
	* use shadow-plane state directly (Noralf)
Signed-off-by: NThomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: NNoralf Trønnes <noralf@tronnes.org>
Tested-by: NJavier Martinez Canillas <javierm@redhat.com>
Tested-by: Noralf Trønnes <noralf@tronnes.org> # drm/tiny/mi0283qt
Link: https://patchwork.freedesktop.org/patch/msgid/20221202125644.7917-7-tzimmermann@suse.de