arm64: perf: Remove PMU locking

The PMU is disabled and enabled, and the counters are programmed from
contexts where interrupts or preemption is disabled.

The functions to toggle the PMU and to program the PMU counters access the
registers directly and don't access data modified by the interrupt handler.
That, and the fact that they're always called from non-preemptible
contexts, means that we don't need to disable interrupts or use a spinlock.

Cc: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Tested-by: Sumit Garg <sumit.garg@linaro.org> (Developerbox)
[Alexandru E.: Explained why locking is not needed, removed WARN_ONs]
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>