gpu: ion: Check return code of msm_allocate_iova_address

In the IOMMU heap, the return value of msm_allocate_iova_address
needs to be checked instead of the value of the iova address.
Check the return value instead.

Change-Id: I4554515d17c926a7dd738824aff21eb65d2ba56a
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>

Conflicts:

	drivers/gpu/ion/ion_iommu_heap.c