ecore_evas: fix to notify mainthread just once

ecore_evas: fix to notify mainthread just once

in tizen, there is an indivisual thread for getting vsync signal.
and the thread notifies the main thread every single time it gets vsync signal.

so that main thread has a possibility to handle multiple times of thread callbacks
(notify handler) when main thread can't iterate within about 16ms
which is an interval of the vsync signal.

this patch fixes the vsync logic not to notify the main thread
when the previous vsync is not handled.

Change-Id: I650a6213f7935f00600496aaac398858f5e9ebf2