drm: Don't set the plane->fb to NULL on successfull set_plane

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: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Rob Clark <rob@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>