drm/etnaviv: use 'sync points' for performance monitor requests

With 'sync points' we can sample the reqeustes perform signals
before and/or after the submited command buffer.

Changes v2 -> v3:
- fixed indentation and init nr_events to 1

Changes v4 -> v5:
- simplify logic around fence handling.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>