drm/etnaviv: add infrastructure to query perf counter

Make it possible that userspace can query all performance domains and
its signals. This information is needed to sample those signals via
submit ioctl.

At the moment no performance domain is available.

Changes from v1 -> v2:
- use a 16 bit value for signals
- fix padding issues
- add id member to domain and signal struct

Changes v4 -> v5
- provide for each pipe an own set of pm domains

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