drm/i915: Fix IVB GT2 WaDisableDopClockGating and WaDisablePSDDualDispatchEnable

IVB GT2 has two registers for these things, and both must be written.

To add a bit more confusion both Bspec and the W/A database state that
WaDisablePSDDualDispatchEnable is only needed for IVB GT1, but the W/A
database also says to write even the second GT2 only register. So I
don't really know what the right thing here is.

Note that Bspec disagrees with the w/a database here, but Ville
confirmed (by asking Chris) that on gt1 the 2nd reg doesn't exist.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Add note as requested by Rodrigo.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -4825,9 +4825,13 @@ static void ivybridge_init_clock_gating(struct drm_device *dev)
 	if (IS_IVB_GT1(dev))
 		I915_WRITE(GEN7_HALF_SLICE_CHICKEN1,
 			   _MASKED_BIT_ENABLE(GEN7_PSD_SINGLE_PORT_DISPATCH_ENABLE));
-	else
+	else {
+		/* must write both registers */
+		I915_WRITE(GEN7_HALF_SLICE_CHICKEN1,
+			   _MASKED_BIT_ENABLE(GEN7_PSD_SINGLE_PORT_DISPATCH_ENABLE));
 		I915_WRITE(GEN7_HALF_SLICE_CHICKEN1_GT2,
 			   _MASKED_BIT_ENABLE(GEN7_PSD_SINGLE_PORT_DISPATCH_ENABLE));
+	}
 
 	/* Apply the WaDisableRHWOOptimizationForRenderHang:ivb workaround. */
 	I915_WRITE(GEN7_COMMON_SLICE_CHICKEN1,
@@ -4841,10 +4845,13 @@ static void ivybridge_init_clock_gating(struct drm_device *dev)
 	if (IS_IVB_GT1(dev))
 		I915_WRITE(GEN7_ROW_CHICKEN2,
 			   _MASKED_BIT_ENABLE(DOP_CLOCK_GATING_DISABLE));
-	else
+	else {
+		/* must write both registers */
+		I915_WRITE(GEN7_ROW_CHICKEN2,
+			   _MASKED_BIT_ENABLE(DOP_CLOCK_GATING_DISABLE));
 		I915_WRITE(GEN7_ROW_CHICKEN2_GT2,
 			   _MASKED_BIT_ENABLE(DOP_CLOCK_GATING_DISABLE));
-
+	}
 
 	/* WaForceL3Serialization:ivb */
 	I915_WRITE(GEN7_L3SQCREG4, I915_READ(GEN7_L3SQCREG4) &