ecore_drm2: fix seat matching when checking for a device with a null seat

Summary: Check to make sure that the seat from the matching device is still null.

Test Plan: Make sure that devices are not being rejected while on seat1. This depends on another patch

Reviewers: ManMower, devilhorns

Reviewed By: devilhorns

Subscribers: ManMower, devilhorns, cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D5711