etnaviv: support performance monitor requests

Add etna_cmd_stream_perf(..) to submit perform requests.
Userspace can submit pmrs via submit ioctl to sample perfmon
signals.

v3:
 - mark perfmon bos as RW

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