drm/etnaviv: add uapi for perfmon feature

Sadly we can not read any registers via command stream so we need
to extend the drm_etnaviv_gem_submit struct with performance monitor
requests. Those requests gets process before or after the actual
submitted command stream.

The Vivante kernel driver has a special ioctl to read all perfmon
registers at once and return it.

Changes from v1 -> v2:
- use a 16 bit value for signals
- fix padding issues

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