drm/i915/chv: Initial clock gating support for Cherryview

drm/i915/chv: Initial clock gating support for Cherryview

CHV clock gating isn't identical to VLV, so add a new function
for it. This is only a start, and further changes are needed as
the details become available.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>