AMD IOMMU: free domain bitmap with its allocation order

The amd_iommu_pd_alloc_bitmap is allocated with a calculated order and
freed with order 1. This is not a bug since the calculated order always
evaluates to 1, but its unclean code. So replace the 1 with the
calculation in the release path.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>