iommu-debug: Use apply_to_new_mapping to reset fastmap

This resolves an issue on arm32 where assigning 1 << 32 to
a size_t causes integer overflow.

Change-Id: Ic6f94873dd6939df9418c111e188418d67bfa6e3
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>