msm: iommu: Avoid using unprobed devices

Fail to look up IOMMU context devices if there is no driver
data associated with them, as this would imply that the
device did not pass the hardware sanity check.

Change-Id: If2998a96dea9342850092344c4ad70eebf965229
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>