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