ecore_evas_extn: update plug image object after disconnection

ecore_evas_extn: update plug image object after disconnection

Summary:
plug image object was not cleaned after server disconnection.
specifically, image data was cleaned (NULL), but it was not updated for real on screen

--> set dirty of plug image object

@fix

Test Plan: elementary_test -> open window socket -> open window plug -> close window socket -> plug should be cleaned up

Reviewers: raster, cedric

Reviewed By: raster

CC: seoz, cedric
Differential Revision: https://phab.enlightenment.org/D703