drm: cleanup: use drm_framebuffer_reference instead of a kref_get

drm_framebuffer_lookup() does a kref_get() for the framebuffer if it finds one
corresponding to the fb id passed to it. Use drm_framebuffer_reference() instead
for clarity since it's the function used in other places to take a reference.

Signed-off-by: Archit Taneja <archit@ti.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>