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>