We need to clear the local variable to get the refcounting right
(since the reference drm_mode_setplane holds is transferred to the
plane->fb pointer). But should be done _after_ we update the pointer.

Breakage introduced in

commit 6c2a7532
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Dec 11 00:59:24 2012 +0100

    drm: refcounting for sprite framebuffers
Reported-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Cc: Rob Clark <rob@ti.com>
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NThierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NDave Airlie <airlied@redhat.com>