etnaviv: add permon support

Query all domains and their signals and provide it this information
via struct etna_perfmon and the corresponding api functions.

v2:
 - code style changes
 - etna_perfmon_create(..): add missing clean up in error case

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