Impact: cleanup, reduce kernel size a bit, avoid sparse warning

Fixes sparse warning:
arch/x86/kernel/amd_iommu_init.c:246:13: warning: symbol 'iommu_enable' was not declared. Should it be static?
arch/x86/kernel/amd_iommu_init.c:259:13: warning: symbol 'iommu_enable_event_logging' was not declared. Should it be static?
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>