drm/vmwgfx: Fix an uninitialized fence handle value

if vmw_execbuf_fence_commands() fails, The handle value will be
uninitialized and a bogus fence handle might be copied to user-space.

Cc: <stable@vger.kernel.org>
Fixes: 2724b2d5: ("drm/vmwgfx: Use new validation interface for the modesetting code v2")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com> #v1
Reviewed-by: Sinclair Yeh <syeh@vmware.com> #v1
Reviewed-by: Deepak Rawat <drawat@vmware.com>