drm/i915: Invalidate media caches on gen7

In the gen7 pipe control there is an extra bit to flush the media
caches, so let's set it during cache invalidation flushes.

v2: Rename to MEDIA_STATE_CLEAR to be more inline with spec.

Cc: Simon Farnsworth <simon@farnz.org.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>