drm/i915: Setup DDI clk for MST on SKL

Set up the DDI->PLL mapping on SKL also for MST links. Might help make
MST operational on SKL.

v2: Rebased due to KBL
    Improve the patch subject, Jesse provided the new one

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1439826380-18403-1-git-send-email-ville.syrjala@linux.intel.com
References: https://bugs.freedesktop.org/show_bug.cgi?id=91791


Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>