drm/i915/bdw: Flush system agent on gen8 also

gem_gtt_cpu_tlb seems to indicate that it is needed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72869

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -906,14 +906,12 @@ static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
 		WARN_ON(readq(&gtt_entries[i-1])
 			!= gen8_pte_encode(addr, level, true));
 
-#if 0 /* TODO: Still needed on GEN8? */
 	/* This next bit makes the above posting read even more important. We
 	 * want to flush the TLBs only after we're certain all the PTE updates
 	 * have finished.
 	 */
 	I915_WRITE(GFX_FLSH_CNTL_GEN6, GFX_FLSH_CNTL_EN);
 	POSTING_READ(GFX_FLSH_CNTL_GEN6);
-#endif
 }
 
 /*