drm/i915: don't use gtt_pwrite on LLC cached objects

~120 µs instead fo ~210 µs to write 1mb on my snb. I like this.
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -808,6 +808,7 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
 	}
 
 	if (obj->gtt_space &&
+	    obj->cache_level == I915_CACHE_NONE &&
 	    obj->base.write_domain != I915_GEM_DOMAIN_CPU) {
 		ret = i915_gem_object_pin(obj, 0, true);
 		if (ret)