client: add mutex lock/unlock for proxy

client: add mutex lock/unlock for proxy

Actually this is not the perfect solution to protect wayland client resources in
multi-threads. They SHOULD be protected by applications. For example, during polling
the display fd, if a thread calls wl_display_disconnect, it will make issues. However,
adding mutex lock/unlock looks worth to protect wayland client resources inside of
wayland library at least.

Change-Id: I26b5f9598ccaaf5620a64a84af49f8637a360acb