intel: Set the region's tiling to none when attaching a PBO to a region.

Note that when detaching the PBO from the region and making a new BO
for the region, we don't make it tiled even if the region originally
was.

Fixes piglit pbo-teximage-tiling.

src/mesa/drivers/dri/intel/intel_regions.c

@@ -442,6 +442,7 @@ intel_region_attach_pbo(struct intel_context *intel,
    region->pbo->region = region;
    dri_bo_reference(buffer);
    region->buffer = buffer;
+   region->tiling = I915_TILING_NONE;
 }