drm/i915: Allow disabling the destination colorkey for overlay

Sometimes userspace wants a true overlay that is never clipped. In such
cases, we need to disable the destination colorkey. However, it is
currently unconditionally enabled in the overlay with no means of
disabling. So rectify that by always default to on, and extending the
UPDATE_ATTR ioctl to support explicit disabling of the colorkey.

This is contrast to the spite code which requires explicit enabling of
either the destination or source colorkey. Handling source colorkey is
still todo for the overlay. (Of course it may be worth migrating overlay
to sprite before then.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>