drm/etnaviv: hold GPU lock across perfmon sampling

The perfmon sampling mutates shared GPU state (e.g. VIVS_HI_CLOCK_CONTROL
to select the pipe for the perf counter reads). To avoid clashing with
other functions mutating the same state (e.g. etnaviv_gpu_update_clock)
the perfmon sampling needs to hold the GPU lock.

Fixes: 68dc0b29 ("drm/etnaviv: use 'sync points' for performance monitor requests")
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>