perf cs-etm: Avoid printing warning in cs_etm_is_ete() check

When checking for the presence of ETE, a register is read that may not
be present on older kernels or if ETE isn't available. cs_etm_get_ro()
will print a warning if it doesn't exist, so check for the existence
first before accessing it.

Reviewed-by: Leo Yan <[email protected]>
Signed-off-by: James Clark <[email protected]>
Cc: Al Grant <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: John Garry <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Poirier <[email protected]>
Cc: Mike Leach <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Suzuki Poulouse <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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