drm/i915: Fix DSPCLK_GATE_D for VLV

Fix the DSPCLK_GATE_D access for VLV. The code incorrectly tried to
poke at the ILK+ version of the register which is at the wrong offset.

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>