powerpc/mpic: Cleanup mpic_find() implementation

mpic_find() was overloaded to do two things, finding the mpic instance
for a given interrupt and returning if it's an IPI. Instead we introduce
mpic_is_ipi() and simplify mpic_find() to just return the mpic instance

Also silences the warning:
arch/powerpc/sysdev/mpic.c: In function 'mpic_irq_set_priority':
arch/powerpc/sysdev/mpic.c:1382: warning: 'is_ipi' may be used uninitialized in this function

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Acked-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>