drm/vmwgfx: constify vmw_fence_ops

vmw_fence_ops are not supposed to change at runtime. Functions
"dma_fence_init" working with const vmw_fence_ops provided
by <linux/dma-fence.h>. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_fence.c

@@ -224,7 +224,7 @@ out:
 	return ret;
 }
 
-static struct dma_fence_ops vmw_fence_ops = {
+static const struct dma_fence_ops vmw_fence_ops = {
 	.get_driver_name = vmw_fence_get_driver_name,
 	.get_timeline_name = vmw_fence_get_timeline_name,
 	.enable_signaling = vmw_fence_enable_signaling,