gpu/drm: ingenic: Avoid null pointer deference in plane atomic update

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


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Acked-by: Sam Ravnborg <sam@ravnborg.org>