e_comp_wl: Dispatch idle sources before going to sleep

e_comp_wl: Dispatch idle sources before going to sleep

Now that we take advantage of libds, which adds idle sources to
wl_event_loop for its own purposes as a building block for wayland
compositor, we need to dispatch wl_event_loop's idle sources before
going to sleep.  Otherwise, we have no chance of dispatching idle
sources until the process awake from sleep.

There must not be side effects after applying this patch, because idle
sources never used by E as far as I know.

Change-Id: Id24b7444b68353d1011f1386158e3c78ddec9d66