drm/i915/mst: fix pipe and vblank enable

Commit 21fd23ac ("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: 21fd23ac ("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: NStanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: NVandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: NStanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Tested-by: NStanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200205082959.31317-1-jani.nikula@intel.com

drivers/gpu/drm/i915/display/intel_dp_mst.c

@@ -491,6 +491,12 @@ static void intel_mst_enable_dp(struct intel_encoder *encoder,
 	struct intel_dp *intel_dp = &intel_dig_port->dp;
 	struct drm_i915_private *dev_priv = to_i915(encoder->base.dev);
 
+	drm_WARN_ON(&dev_priv->drm, pipe_config->has_pch_encoder);
+
+	intel_enable_pipe(pipe_config);
+
+	intel_crtc_vblank_on(pipe_config);
+
 	DRM_DEBUG_KMS("active links %d\n", intel_dp->active_mst_links);
 
 	if (intel_de_wait_for_set(dev_priv, intel_dp->regs.dp_tp_status,