x86/events/amd/iommu: Make iommu_pmu const and __initconst

iommu_pmu is only used as source for a copy operation in the init code
path.

Mark it const and __initconst.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: julia.lawall@lip6.fr
Link: https://lkml.kernel.org/r/1505819443-670-1-git-send-email-bhumirks@gmail.com