drm/vkms: Inherit plane state from struct drm_shadow_plane_state

Subclass struct drm_shadow_plane_state for VKMS planes and update
all plane-state callbacks accordingly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210705074633.9425-3-tzimmermann@suse.de