Don't access plane_state->fb until we know the plane to be visible.
It it's visible, it will have an fb, and thus we don't have to
consider the NULL fb case. Makes the code look nicer.

Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-25-git-send-email-ville.syrjala@linux.intel.comReviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>