drm/i915: Disable DPOunit clock gating

Otherwise, this can stall pipe. We also need DPLL REFA always
enabled

Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>