drm/i915: track sprite coverage and disable primary plane if possible

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: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>