e_comp_wl: clear comp object immediately instead of unref e_client

e_comp_wl: clear comp object immediately instead of unref e_client

When a buffer of deleted e_client destroied, it has unrefed e_client and freed
e_client. but if there remains some action using the e_client such as ecore
event, it's able to try to use e_client which is freed before. for example, if
the event free function is called after buffer destroy, it will try to unref
e_client which is freed already.

For prevent those problem, it's desirable to clear comp object immediately
insted of to unref e_client.

related patch: feaa183a35fbbae4fcf6b9cf27eaee3e7432db73

Change-Id: I47c971b5ae1d1a5ff3a6c11bb5e80ad21c9b9224
Signed-off-by: MinJeong Kim <minjjj.kim@samsung.com>