msm: kgsl: Fix checking of returned error

IOMMU get context functions return error as a pointer so just
checking for NULL is not correct. Use the right macro to check
if error is returned and only force initialization of one context
for IOMMU-v1 since other contexts are not used.

Change-Id: I5123363567649568127e24738f19ea30f98a6cec
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>