drm/i915: Drop wmb() inside pread_gtt

Inside pread, we only ever read from the GTT so the serialising wmb()
instructions around the GGTT PTE updates are pointless.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718145407.21352-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -395,11 +395,9 @@ i915_gem_gtt_pread(struct drm_i915_gem_object *obj,
 		unsigned page_length = PAGE_SIZE - page_offset;
 		page_length = remain < page_length ? remain : page_length;
 		if (node.allocated) {
-			wmb();
 			ggtt->vm.insert_page(&ggtt->vm,
 					     i915_gem_object_get_dma_address(obj, offset >> PAGE_SHIFT),
 					     node.start, I915_CACHE_NONE, 0);
-			wmb();
 		} else {
 			page_base += offset & PAGE_MASK;
 		}
@@ -419,7 +417,6 @@ i915_gem_gtt_pread(struct drm_i915_gem_object *obj,
 out_unpin:
 	mutex_lock(&i915->drm.struct_mutex);
 	if (node.allocated) {
-		wmb();
 		ggtt->vm.clear_range(&ggtt->vm, node.start, node.size);
 		remove_mappable_node(&node);
 	} else {