drm: Add additional atomic helpers for shadow-buffered planes

Several drivers use GEM buffer objects as shadow buffers for the actual
framebuffer memory. Right now, drivers do these vmap operations in their
commit tail, which is actually not allowed by the locking rules for
the dma-buf reservation lock. The involved BO has to be vmapped in the
plane's prepare_fb callback and vunmapped in cleanup_fb.

This patch introduces atomic helpers for such shadow planes. Plane
functions manage the plane state for shadow planes. The provided
implementations for prepare_fb and cleanup_fb vmap and vunmap all BOs of
struct drm_plane_state.fb. The mappings are afterwards available in the
plane's commit-tail functions.

For now, all rsp drivers use the simple KMS helpers, so we add the plane
callbacks and wrappers for simple KMS. The internal plane functions can
later be exported as needed.

v3:
	* documentation fixes
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/20210208115538.6430-4-tzimmermann@suse.de