e_client: check buffer_flush flag in e_client_uniconify

e_client: check buffer_flush flag in e_client_uniconify

There was a bug that the visibility toggled on/off as below scenario.
1. WinA set buffer_flush and deiconify_update to 0.
2. WinA attached Null buffer.
3. Turn off the screen.
4. Turn on the screen.
5. WinA's visibility was toggled on and off continuously

This was because we didn't consider buffer_flush and deiconify_update states in
e_client_uniconify.
So, we add code to check the window's buffer_flush and deiconify_update state.
If those values are set to all 0, then e shows the window in e_client_uniconify.

Change-Id: I32e030689c04608510783df367d9c30657e8777b