drm/i915: Use FORCEWAKE_KERNEL instead of hardcoded number in MT forcewake ACK

The MT forcewake ACK register also has a corresponding bit to each of
the bits in the MT forcewake register. Use the define we have for the
bit we care about instead of a hardcoded number.

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