Comment 1 for bug 1515169

I've backported this commit:

commit e404ba8d06ff2a1bdb916a9e5d2c09cacd7e5ca3
Author: Ville Syrjälä <email address hidden>
Date: Mon Aug 17 18:46:20 2015 +0300

    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 <email address hidden>
    Signed-off-by: Ville Syrjälä <email address hidden>
    Link: http://patchwork.freedesktop<email address hidden>
    References: https://bugs.freedesktop.org/show_bug.cgi?id=91791
    Reviewed-by: Jesse Barnes <email address hidden>

for our i915_bpo driver, and it has been tested to fix the hang.