perf list: For metricgroup only list include description

If perf list is invoked with 'metricgroups' include the description
unless it is invoked with flags to exclude it. Make the description of
metricgroup dumping dependent on the desc flag in print_state as with
metrics.

Before:
```
$ perf list metricgroups
List of pre-defined events (to be used in -e or -M):

Metric Groups:

Backend
Bad
BadSpec
...
```

After:
```
$ perf list metricgroups
List of pre-defined events (to be used in -e or -M):

Metric Groups:

Backend [Grouping from Top-down Microarchitecture Analysis Metrics spreadsheet]
Bad [Grouping from Top-down Microarchitecture Analysis Metrics spreadsheet]
BadSpec
...
```

Signed-off-by: Ian Rogers <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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