perf pfm: Don't force loading of all PMUs

perf pfm: Don't force loading of all PMUs

Force loading all PMUs adds significant cost because DRM and other
PMUs are loaded, it should also not be required if the pmus__
functions are used.

Tested by run perf test, in particular the pfm related tests. Also
`perf list` is identical before and after.

Before:
  $ time ./perf test pfm
   54: Test libpfm4 support                                            :
   54.1: test of individual --pfm-events                               : Ok
   54.2: test groups of --pfm-events                                   : Ok
  103: perf all libpfm4 events test                                    : Ok

  real	0m8.933s
  user	0m1.824s
  sys	0m7.122s

After:
  $ time ./perf test pfm
   54: Test libpfm4 support                                            :
   54.1: test of individual --pfm-events                               : Ok
   54.2: test groups of --pfm-events                                   : Ok
  103: perf all libpfm4 events test                                    : Ok

  real	0m5.259s
  user	0m1.793s
  sys	0m3.570s

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20250722013449.146233-1-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>