mesa/main: allow delayed initialization of performance monitors

mesa/main: allow delayed initialization of performance monitors

Most applications never use performance counters, so allow drivers to
skip potentially expensive initialization steps.

A driver that wants to use this must enable the appropriate extension(s)
at context initialization and set the InitPerfMonitorGroups driver function
which will be called the first time information about the performance monitor
groups is actually used.

The init_groups helper is called for API functions that can be called before
a monitor object exists. Functions that require an existing monitor object
can rely on init_groups having been called before.

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