To save power when the sprite is full screen, we can disable the primary
plane on the same pipe.  Track the sprite status and enable/disable the
primary opportunistically.

v2: remove primary plane enable/disable hooks; they're identical
Reviewed-by: NDaniel Vetter <daniel@ffwll.ch>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NKeith Packard <keithp@keithp.com>