drm/etnaviv: hook up DRM GPU scheduler

This hooks in the DRM GPU scheduler. No improvement yet, as all the
dependency handling is still done in etnaviv_gem_submit. This just
replaces the actual GPU submit by passing through the scheduler.

Allows to get rid of the retire worker, as this is now driven by the
scheduler.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>