drm/i915: Unify VLV/CHV DPOunit clock gating disable/enable

drm/i915: Unify VLV/CHV DPOunit clock gating disable/enable

Check for VLV/CHV instead if !BXT when re-enabling DPOunit clock gating
after DSI disable. That's what we checked when disabling the clock
gating when enabling DSI.

Also use the same temporary variable name in both cases, and toss in a
bit of dev vs. dev_priv cleanup while at it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460996305-30453-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>