intel-iommu: fix build error with INTR_REMAP=y and DMAR=n

dmar.o can be built in the CONFIG_INTR_REMAP=y case but
iommu_calculate_agaw() is only available if VT-d is built as well.

So create an inline version of iommu_calculate_agaw() for the
!CONFIG_DMAR case. The iommu->agaw value wont be used in this
case, but the code is cleaner (has less #ifdefs) if we have it around
unconditionally.

Signed-off-by: Ingo Molnar <mingo@elte.hu>