Revert "drm/i915: enable HiZ Raw Stall Optimization on IVB"

This reverts commit 116f2b6d.

This optimization causes widespread corruption in games, and even in
glxgears, on my ivb:gt1. The corruption appears like z-fighting of
overlapping polygons in the HiZ buffer.

The observation ties in very closely with the description of the
optimization disabled by default on IVB:

"The Hierarchical Z RAW Stall Optimization allows non-overlapping
polygons in the same 8x4 pixel/sample area to be processed without
stalling waiting for the earlier ones to write to Hierarchical Z
buffer."

No reason is given for why it is disabled by default, usually for such
optimizations it is that it is incomplete. However, there is no
indication whether this a gt1 only issue either. Before considering
reenabling this optimization, I would first suggest reproducing the
corruption in piglit.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75623Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Chia-I Wu <olv@lunarg.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -4972,9 +4972,11 @@ static void ivybridge_init_clock_gating(struct drm_device *dev)
 
 	gen7_setup_fixed_func_scheduler(dev_priv);
 
+	if (0) { /* causes HiZ corruption on ivb:gt1 */
 		/* enable HiZ Raw Stall Optimization */
 		I915_WRITE(CACHE_MODE_0_GEN7,
 			   _MASKED_BIT_DISABLE(HIZ_RAW_STALL_OPT_DISABLE));
+	}
 
 	/* WaDisable4x2SubspanOptimization:ivb */
 	I915_WRITE(CACHE_MODE_1,