drm/i915: implement WADP0ClockGatingDisable

Found in Bspec vol4h South Display Engine Registers [CPT, PPT],
section "5.3.1  TRANS_CHICKEN_1—Transcoder Chicken Bits 1"

v2: Make it compile.
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -3824,6 +3824,10 @@
 #define  TRANS_6BPC             (2<<5)
 #define  TRANS_12BPC            (3<<5)
 
+#define _TRANSA_CHICKEN1	 0xf0060
+#define _TRANSB_CHICKEN1	 0xf1060
+#define TRANS_CHICKEN1(pipe) _PIPE(pipe, _TRANSA_CHICKEN1, _TRANSB_CHICKEN1)
+#define  TRANS_CHICKEN1_DP0UNIT_GC_DISABLE	(1<<4)
 #define _TRANSA_CHICKEN2	 0xf0064
 #define _TRANSB_CHICKEN2	 0xf1064
 #define TRANS_CHICKEN2(pipe) _PIPE(pipe, _TRANSA_CHICKEN2, _TRANSB_CHICKEN2)

drivers/gpu/drm/i915/intel_pm.c

@@ -3794,6 +3794,7 @@ static void ibx_init_clock_gating(struct drm_device *dev)
 static void cpt_init_clock_gating(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
+	int pipe;
 
 	/*
 	 * On Ibex Peak and Cougar Point, we need to disable clock
@@ -3803,6 +3804,11 @@ static void cpt_init_clock_gating(struct drm_device *dev)
 	I915_WRITE(SOUTH_DSPCLK_GATE_D, PCH_DPLSUNIT_CLOCK_GATE_DISABLE);
 	I915_WRITE(SOUTH_CHICKEN2, I915_READ(SOUTH_CHICKEN2) |
 		   DPLS_EDP_PPS_FIX_DIS);
+	/* WADP0ClockGatingDisable */
+	for_each_pipe(pipe) {
+		I915_WRITE(TRANS_CHICKEN1(pipe),
+			   TRANS_CHICKEN1_DP0UNIT_GC_DISABLE);
+	}
 }
 
 void intel_init_clock_gating(struct drm_device *dev)