iommu/arm-smmu: Use an atomic context when fastmap is set on GKI

By default fastmap uses an atomic context.
On GKI builds fastmap is not supported, but we need to ensure that when
a client requests that fastmap be enabled on a GKI build that they still
use an atomic context.

Change-Id: I145e4679e973b10f666a0bef6ba104223fa5e7a9
Signed-off-by: Liam Mark <lmark@codeaurora.org>