drm/i915: set w/a bit for snb pagefaults

Bspec says that we need to set this: vol1c.3 "Blitter Command
Streamer", Section 1.1.2.1 "GAB_CTL_REG - GAB Unit Control Register".

We don't really rely on pagefaults, but who knows what this all
affects.
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
Signed-Off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -3669,7 +3669,12 @@ void i915_gem_init_ppgtt(struct drm_device *dev)
 	pd_offset <<= 16;
 
 	if (INTEL_INFO(dev)->gen == 6) {
-		uint32_t ecochk = I915_READ(GAM_ECOCHK);
+		uint32_t ecochk, gab_ctl;
+
+		gab_ctl = I915_READ(GAB_CTL);
+		I915_WRITE(GAB_CTL, gab_ctl | GAB_CTL_CONT_AFTER_PAGEFAULT);
+
+		ecochk = I915_READ(GAM_ECOCHK);
 		I915_WRITE(GAM_ECOCHK, ecochk | ECOCHK_SNB_BIT |
 				       ECOCHK_PPGTT_CACHE64B);
 		I915_WRITE(GFX_MODE, GFX_MODE_ENABLE(GFX_PPGTT_ENABLE));

drivers/gpu/drm/i915/i915_reg.h

@@ -127,6 +127,9 @@
 #define   ECOCHK_PPGTT_CACHE64B		(0x3<<3)
 #define   ECOCHK_PPGTT_CACHE4B		(0x0<<3)
 
+#define GAB_CTL				0x24000
+#define   GAB_CTL_CONT_AFTER_PAGEFAULT	(1<<8)
+
 /* VGA stuff */
 
 #define VGA_ST01_MDA 0x3ba