msm: iommu: Use the correct memory allocation flag

Change msm_iommu_map to use GFP_ATOMIC instead of
GFP_KERNEL due to the fact that the call occurs within
a spinlock-protected region.

Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>