drm/i915/mst: fix pipe and vblank enable

drm/i915/mst: fix pipe and vblank enable

Commit 9cd1777cd59e ("drm/i915: move pipe, pch and vblank enable to
encoders on DDI platforms") pushed pipe and vblank enable to encoders on
DDI platforms, however it missed the DP MST encoder. Fix it.

Fixes: 9cd1777cd59e ("drm/i915: move pipe, pch and vblank enable to encoders on DDI platforms")
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reported-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Tested-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200205082959.31317-1-jani.nikula@intel.com