drm/i915: Implement WaVcpClkGateDisableForMediaReset:ctg, elk

Apparently we need to disable VCP unit clock gating around media reset
on g4x.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>