drm/vmwgfx: unlock on error path in vmw_execbuf_process()

There is a missing unlock on error here.

Fixes: 30f82d81 ('drm/vmwgfx: Reemit context bindings when necessary v2')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>