drm/i915: add color key support v4

Add new ioctls for getting and setting the current destination color
key.  This allows for simple overlay display control by matching a color
key value in the primary plane before blending the overlay on top.

v2: remove unnecessary mutex acquire/release around reg accesses
v3: add support for full color key management
v4: fix copy & paste bug in snb_get_colorkey
    don't bother checking min/max values against docs as the docs are likely
    wrong (how could we handle 10bpc surface formats?)

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>