drm/i915: Setup DPLL/DPLLMD for DSI too on VLV/CHV

drm/i915: Setup DPLL/DPLLMD for DSI too on VLV/CHV

Set up DPLL and DPLL_MD even when driving DSI output on VLV/CHV. While
the DPLL isn't used to provide the clock we still need the refclock, and
it appears that the pixel repeat factor also has an effect on DSI
output. So set up eveyrhing in DPLL and DPLL_MD as we would do for
DP/HDMI/VGA, but don't actually enable the DPLL or configure the
dividers via DPIO.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460488478-18311-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Jani Nikula <jani.nikula@intel.com>