drm/i915/bxt: Restrict PORT_CLK_SEL programming below gen9

PORT_CLK_SEL programming is needed only on HSW/BDW.

v2:
- don't program PORT_CLK_SEL from mst encoders either (imre)
v3:
- fix the check for GEN9+ in intel_mst_pre_enable_dp() (damien)

Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sagar Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
index 7335089..3945057 100644
--- a/drivers/gpu/drm/i915/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/intel_dp_mst.c
@@ -173,8 +173,10 @@
 	if (intel_dp->active_mst_links == 0) {
 		enum port port = intel_ddi_get_encoder_port(encoder);
 
-		I915_WRITE(PORT_CLK_SEL(port),
-			   intel_crtc->config->ddi_pll_sel);
+		/* FIXME: add support for SKL */
+		if (INTEL_INFO(dev)->gen < 9)
+			I915_WRITE(PORT_CLK_SEL(port),
+				   intel_crtc->config->ddi_pll_sel);
 
 		intel_ddi_init_dp_buf_reg(&intel_dig_port->base);