msm: camera: smmu: Avoid NULL dereference in cam_smmu_alloc_firmware

The iommu domain we grab from iommu_get_domain_for_dev can
be NULL. Eventually we will hit a NULL dereference in the
cam_smmu_alloc_firmware due to the missing check. Modify the
existing check to account for the possiblity of a NULL
domain and bail early.

CRs-Fixed: 2804766
Change-Id: I201260c3559b847cfb0bf7f85226816af148671d
Signed-off-by: Fernando Pacheco <fpacheco@codeaurora.org>