Revert "drm/i915: W/A for underruns with WM1+ disabled on icl"

This reverts commit bf002c10.

The hw team has come up with a better workaround. So
let's get rid of this one.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190213165424.22904-1-ville.syrjala@linux.intel.comReviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -7653,7 +7653,6 @@ enum {
 #define _PIPEB_CHICKEN			0x71038
 #define _PIPEC_CHICKEN			0x72038
 #define  PER_PIXEL_ALPHA_BYPASS_EN	(1 << 7)
-#define  PM_FILL_MAINTAIN_DBUF_FULLNESS	(1 << 0)
 #define PIPE_CHICKEN(pipe)		_MMIO_PIPE(pipe, _PIPEA_CHICKEN,\
 						   _PIPEB_CHICKEN)
 

drivers/gpu/drm/i915/intel_display.c

@@ -3962,12 +3962,6 @@ static void icl_set_pipe_chicken(struct intel_crtc *crtc)
 	 */
 	tmp |= PER_PIXEL_ALPHA_BYPASS_EN;
 
-	/*
-	 * W/A for underruns with linear/X-tiled with
-	 * WM1+ disabled.
-	 */
-	tmp |= PM_FILL_MAINTAIN_DBUF_FULLNESS;
-
 	I915_WRITE(PIPE_CHICKEN(pipe), tmp);
 }