drm/i915: Program iboost settings for HDMI/DVI on SKL

Currently we fail to program the iboost stuff for HDMI/DVI. Let's remedy
that.

Cc: stable@vger.kernel.org
Fixes: f8896f5d ("drm/i915/skl: Buffer translation improvements")
Cc: David Weinehall <david.weinehall@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468328376-6380-4-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
(cherry picked from commit 8d8bb85e)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>