drm/etnaviv: move runtime PM handling to events

Conceptually events are the right abstraction to handle the GPU
runtime PM state: as long as any event is pending the GPU can not
be idle. Events are also properly freed and reallocated when the
GPU has been reset after a hang.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>