drm/vboxvideo: Implement cursor plane with struct drm_shadow_plane_state

Functions in the atomic commit tail are not allowed to acquire the
dmabuf's reservation lock. So we cannot legally call the GEM object's
vmap functionality in atomic_update.

Instead use struct drm_shadow_plane_state and friends. It vmaps the
framebuffer BOs in prepare_fb and vunmaps them in cleanup_fb. The
cursor plane state stores the mapping's address. The pinning of the
BO is implicitly done by vmap.

As an extra benefit, there's no source of runtime errors left in
atomic_update.

v2:
	* rebase patch onto struct drm_shadow_plane_state

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209121042.24098-3-tzimmermann@suse.de