MIPS: cevt-r4k: Move handle_perf_irq() out of header

MIPS: cevt-r4k: Move handle_perf_irq() out of header

Long ago, commit 89b41fe18e0e ("[MIPS] SMTC: Fix SMTC dyntick support.")
moved handle_perf_irq() out of cevt-r4k.c into a header so it could be
shared with cevt-smtc.c.

Slightly less long ago, commit a2b06a2bf41f ("MIPS: MT: Remove SMTC
support") removed all traces of SMTC support, including cevt-smtc.c,
leaving cevt-r4k.c once again the sole user of handle_perf_irq(),
therefore move it back into cevt-r4k.c from the header.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9123/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>