vmwgfx: Break out and comment vmw_execbuf_copy_fence_user

This function will be used also by the upcoming fence event code,
so break it out and add a comment about the functionality.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>