drm/i915: Remove superfluous posting reads after clear GGTT

The barrier here is not required - we apply the barrier before the range
is ever reused by the GPU instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-16-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -2184,7 +2184,6 @@ static void gen8_ggtt_clear_range(struct i915_address_space *vm,
 
 	for (i = 0; i < num_entries; i++)
 		gen8_set_pte(&gtt_base[i], scratch_pte);
-	readl(gtt_base);
 }
 
 static void gen6_ggtt_clear_range(struct i915_address_space *vm,
@@ -2209,7 +2208,6 @@ static void gen6_ggtt_clear_range(struct i915_address_space *vm,
 
 	for (i = 0; i < num_entries; i++)
 		iowrite32(scratch_pte, &gtt_base[i]);
-	readl(gtt_base);
 }
 
 static void i915_ggtt_insert_page(struct i915_address_space *vm,
@@ -2233,7 +2231,6 @@ static void i915_ggtt_insert_entries(struct i915_address_space *vm,
 		AGP_USER_MEMORY : AGP_USER_CACHED_MEMORY;
 
 	intel_gtt_insert_sg_entries(pages, start >> PAGE_SHIFT, flags);
-
 }
 
 static void i915_ggtt_clear_range(struct i915_address_space *vm,