client: Make wl_proxy_set_queue() with NULL revert to default queue

This will be useful in order to implement the
EGL_WL_create_wayland_buffer_from_image extension. The buffers created
within Mesa's Wayland platform are created using the the wl_drm object
as a proxy factory which means they will be set to use Mesa's internal
event queue. However, these buffers will be owned by the client
application so they ideally need to use the default event loop. This
function provides a way to set the proxy's event queue back to the
default.

krh: Edited from Neils original patch to just use wl_proxy_set_queue() with
a NULL argument instead of introducing a new function.