drm/i915: Remove the WaDisableBackToBackFlipFix w/a for Haswell

This workaround is only valid for IVB and VLV and the write triggers an
error on HSW.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanonI@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -3474,10 +3474,6 @@ static void haswell_init_clock_gating(struct drm_device *dev)
 	I915_WRITE(_3D_CHICKEN3,
 		   _MASKED_BIT_ENABLE(_3D_CHICKEN_SF_DISABLE_OBJEND_CULL));
 
-	I915_WRITE(IVB_CHICKEN3,
-		   CHICKEN3_DGMG_REQ_OUT_FIX_DISABLE |
-		   CHICKEN3_DGMG_DONE_FIX_DISABLE);
-
 	/* Apply the WaDisableRHWOOptimizationForRenderHang workaround. */
 	I915_WRITE(GEN7_COMMON_SLICE_CHICKEN1,
 		   GEN7_CSC1_RHWO_OPT_DISABLE_IN_RCC);