drm/etnaviv: copy pmrs from userspace

Changes from v1 -> v2:
- renamed submit_perfmon_request() to submit_perfmon_validate()
- extended flags validation
- added comment about offset 0
- moved assigment of cmdbuf->nr_pmrs below the copy_from_user of the pmrs.

Changes from v2 -> v3:
- fixed flags validation

Changes v4 -> v5
- pass cmdbuf->exec_state to etnaviv_pm_req_validate(..)

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