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>

drivers/gpu/drm/i915/intel_ddi.c

@@ -1567,7 +1567,7 @@ static void intel_ddi_pre_enable(struct intel_encoder *intel_encoder)
 
 		I915_WRITE(DPLL_CTRL2, val);
 
-	} else {
+	} else if (INTEL_INFO(dev)->gen < 9) {
 		WARN_ON(crtc->config->ddi_pll_sel == PORT_CLK_SEL_NONE);
 		I915_WRITE(PORT_CLK_SEL(port), crtc->config->ddi_pll_sel);
 	}
@@ -1626,7 +1626,7 @@ static void intel_ddi_post_disable(struct intel_encoder *intel_encoder)
 	if (IS_SKYLAKE(dev))
 		I915_WRITE(DPLL_CTRL2, (I915_READ(DPLL_CTRL2) |
 					DPLL_CTRL2_DDI_CLK_OFF(port)));
-	else
+	else if (INTEL_INFO(dev)->gen < 9)
 		I915_WRITE(PORT_CLK_SEL(port), PORT_CLK_SEL_NONE);
 }