msm: iommu: Fix error handling for uninitialized contexts

Do not return a context pointer if the context does not
have driver data associated with it to ensure that IOMMU
functions fail gracefully on targets where the IOMMU
hardware could not be found.

Change-Id: Ibf915251a4a133c2baaf9fb5b01145fb3c419347
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>