msm: camera: Null check added at iommu_attach_device

In case domain has not been initialized before, added a null
check for it inorder to avoid a potential kernel panic.

CRs-Fixed: 502383
Change-Id: I43a173e96415bdaea0968ce6ca37e6c1ee75d666
Signed-off-by: Aditya Jonnalagadda <ajonnala@codeaurora.org>