drm/sti: Stop consulting plane->fb

We want to get rid of plane->fb on atomic drivers. Stop looking at it.

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322152313.6561-12-ville.syrjala@linux.intel.com


Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>