MIPS: perf: Fix build with CONFIG_CPU_BMIPS5000 enabled

arch/mips/kernel/perf_event_mipsxx.c: In function 'mipsxx_pmu_enable_event':
arch/mips/kernel/perf_event_mipsxx.c:326:21: error: unused variable 'event' [-Werror=unused-variable]
  struct perf_event *event = container_of(evt, struct perf_event, hw);
                     ^~~~~

Fix this by making use of IS_ENABLED() to simplify the code and avoid
unnecessary ifdefery.

Fixes: 84002c88 ("MIPS: perf: Fix perf with MT counting other threads")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@linux-mips.org
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: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@vger.kernel.org
Cc: stable@vger.kernel.org # v4.18+