drm/vmwgfx: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: NRoland Scheidegger <sroland@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_surface.c

@@ -1969,7 +1969,7 @@ static int vmw_surface_dirty_alloc(struct vmw_resource *res)
 		num_mip = 1;
 
 	num_subres = num_layers * num_mip;
-	dirty_size = sizeof(*dirty) + num_subres * sizeof(dirty->boxes[0]);
+	dirty_size = struct_size(dirty, boxes, num_subres);
 	acc_size = ttm_round_pot(dirty_size);
 	ret = ttm_mem_global_alloc(vmw_mem_glob(res->dev_priv),
 				   acc_size, &ctx);