backends/drm: refactor surface handling

In order to support layered rendering and tiled outputs KWin needs to be
able to split rendering of outputs into multiple surfaces. This commit
prepares the drm backend for that, by moving most of the code in EglGbmBackend
out to a EglGbmSurface class, which will later be used for overlay surfaces
and rendering to multiple connectors side by side.

In doing that, this commit also cleans up the code a bit, removes a lot of
now unnecessary multi-gpu stuff and potentially makes modesets a little
bit more efficient by re-using resources more often.