drm/msm: correct attempted NULL pointer dereference in put_iova

put_iova() would attempt to dereference a NULL pointer via the
address space pointer when no IOMMU is present. Correct this by adding
the appropriate check.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>