[4.14] Backport request: powerpc/perf: Fix IMC_MAX_PMU macro

Dear stable team

Please backport the following patch.

Commit: 7029d1eb0c2c7ee093dc625c679fc277c8eb623b ("powerpc/perf: Fix IMC_MAX_PMU macro")

Stable tree targeted: 4.14 (applies cleanly)

Justification: This fixes some Oopses observed on boot on one of our POWER9 boxes. Fix was identified by bisection, the other patch in the 2-patch series has already been applied to 4.14 but this one wasn't for some reason.

Thanks,