It is possible that there is no drm_framebuffer associated with a given
plane state.

v2: Handle drm_plane->state which can be NULL too
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210144142.33143-2-paul@crapouillou.net
# *** extracted tags ***
Acked-by: Sam Ravnborg <sam@ravnborg.org>