drm/i915: implement w/a for incorrect guarband clipping

According to Bsepc, this should be set by default, but isn't. See vo1c.4
"Render Engine Command Streamer", Section 1.1.14.3 "3D_CHICKEN3"

Bspec also says that we always need to set all mask bits.

v2: Add comment about the mask bits wtf.
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -497,6 +497,7 @@
  */
 # define _3D_CHICKEN2_WM_READ_PIPELINED			(1 << 14)
 #define _3D_CHICKEN3	0x02090
+#define  _3D_CHICKEN_SF_DISABLE_FASTCLIP_CULL		(1 << 5)
 
 #define MI_MODE		0x0209c
 # define VS_TIMER_DISPATCH				(1 << 6)

drivers/gpu/drm/i915/intel_display.c

@@ -8897,6 +8897,10 @@ static void gen6_init_clock_gating(struct drm_device *dev)
 		   GEN6_RCPBUNIT_CLOCK_GATE_DISABLE |
 		   GEN6_RCCUNIT_CLOCK_GATE_DISABLE);
 
+	/* Bspec says we need to always set all mask bits. */
+	I915_WRITE(_3D_CHICKEN, (0xFFFF << 16) |
+		   _3D_CHICKEN_SF_DISABLE_FASTCLIP_CULL);
+
 	/*
 	 * According to the spec the following bits should be
 	 * set in order to enable memory self-refresh and fbc: