drm/vmwgfx: Add export fence to file descriptor support

Added code to link a fence to a out_fence_fd file descriptor and
thread out_fence_fd down to vmw_execbuf_copy_fence_user() so it can be
copied into the IOCTL reply and be passed back up the the user.

v2:
Make sure to sync and clean up in case of failure

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Deepak Singh Rawat <drawat@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>