drm/i915: Fix iboost setting for DDI with 4 lanes on SKL

Bspec says:
"For DDIA with x4 capability (DDI_BUF_CTL DDIA Lane Capability Control =
 DDIA x4), the I_boost value has to be programmed in both
 tx_blnclegsctl_0 and tx_blnclegsctl_4."

Currently we only program tx_blnclegsctl_0. Let's do the other one as
well.

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/1468328376-6380-2-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
(cherry picked from commit a7d8dbc0)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>