drm/etnaviv: extend etnaviv_gpu_cmdbuf_new(..) with nr_pmrs

This commits extends etnaviv_gpu_cmdbuf_new(..) to define the number
of struct etnaviv_perfmon elements gets used.

Changes from v1 -> v2:
- make use of goto as requested by Lucas

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