msm: iommu: Check for NULL genpools during domain teardown

During domain creation, some of the genpools will be set to NULL if
there is not enough memory for creation. Check for these NULL
genpools during domain destruction to avoid NULL dereferences.

Change-Id: I60238c138b809ef5c13383f5b1752ab6c799707d
CRs-Fixed: 573612
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>