Emit mode-change on all relevant changes

Summary:
The EGL GBM backend must be informed about mode changes so that the surface
gets reset. This fixes mode changes for me. Rotations still not work, this
needs some more structure cleanup.

Test Plan: Manually changed resolution and scale alone and together with KScreen.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23435