drm/i915: enable WaDisableDopClkGating for skl

This WA is required when decoupled frequencies for slice and unslice
are enabled. This disables DOP clock gating for skl.

v2: enable the WA for all gen9 platforms (not just for SKL GT4 where
    the hang issue is originally reported) to avoid rare hangs (David)
v3: as per WaDatabase, enable it only for SKL (Rodrigo)

Cc: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
Signed-off-by: Praveen Paneri <praveen.paneri@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1501781530-8186-1-git-send-email-praveen.paneri@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -78,6 +78,12 @@ static void gen9_init_clock_gating(struct drm_i915_private *dev_priv)
 	/* WaFbcHighMemBwCorruptionAvoidance:skl,bxt,kbl,cfl */
 	I915_WRITE(ILK_DPFC_CHICKEN, I915_READ(ILK_DPFC_CHICKEN) |
 		   ILK_DPFC_DISABLE_DUMMY0);
+
+	if (IS_SKYLAKE(dev_priv)) {
+		/* WaDisableDopClockGating */
+		I915_WRITE(GEN7_MISCCPCTL, I915_READ(GEN7_MISCCPCTL)
+			   & ~GEN7_DOP_CLOCK_GATE_ENABLE);
+	}
 }
 
 static void bxt_init_clock_gating(struct drm_i915_private *dev_priv)