drm: Don't implement empty prepare_fb()/cleanup_fb()

The plane .prepare_fb() and .cleanup_fb() helpers are optional, there's
no need to implement empty stubs, and no need to explicitly set the
function pointers to NULL either.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[danvet: Resolved conflicts with Chris' patch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>