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>
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index f32a594..3f80816 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -997,6 +997,8 @@
 
 
 /* intel_ddi.c */
+void intel_ddi_clk_select(struct intel_encoder *encoder,
+			  const struct intel_crtc_state *pipe_config);
 void intel_prepare_ddi(struct drm_device *dev);
 void hsw_fdi_link_train(struct drm_crtc *crtc);
 void intel_ddi_init(struct drm_device *dev, enum port port);