perf pmu: When using default config, record which bits of config were changed by the user

perf pmu: When using default config, record which bits of config were changed by the user

Default config for a PMU is defined before selected events are parsed.
That allows the user-entered config to override the default config.

However that does not allow for changing the default config based on
other options.

For example, if the user chooses AUX area sampling mode, in the case of
Intel PT, the psb_period needs to be small for sampling, so there is a
need to set the default psb_period to 0 (2 KiB) in that case. However
that should not override a value set by the user. To allow for that,
when using default config, record which bits of config were changed by
the user.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/20191115124225.5247-13-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>