drm/msm/a6xx: Zap counters across context switch

Any app controlled perfcntr collection (GL_AMD_performance_monitor, etc)
does not require counters to maintain state across context switches.  So
clear them if systemwide profiling is not active.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20220304005317.776110-5-robdclark@gmail.com