drm/i915: properly set ppgtt cacheability on snb

For some reason snb has 2 fields to set ppgtt cacheability. This one
here does not exist on gen7.

This might explain why ppgtt wasn't a win on snb like on ivb - not
enough pte caching.

v2: Fixup rebase fail.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 92acc5f..aa44ff2 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3669,7 +3669,10 @@
 	pd_offset <<= 16;
 
 	if (INTEL_INFO(dev)->gen == 6) {
-		uint32_t ecochk, gab_ctl;
+		uint32_t ecochk, gab_ctl, ecobits;
+
+		ecobits = I915_READ(GAC_ECO_BITS); 
+		I915_WRITE(GAC_ECO_BITS, ecobits | ECOBITS_PPGTT_CACHE64B);
 
 		gab_ctl = I915_READ(GAB_CTL);
 		I915_WRITE(GAB_CTL, gab_ctl | GAB_CTL_CONT_AFTER_PAGEFAULT);