st/mesa: maintain active perfmon counters in an array

st/mesa: maintain active perfmon counters in an array

It is easy enough to pre-determine the required size, and arrays are
generally better behaved especially when they get large.

v2: make sure init_perf_monitor returns true when no counters are active
(spotted by Samuel Pitoiset)

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>