iommu: msm: Use GDSC property for power status check

To get the power status of the SMMUs, we were relying upon
number of context bank already attached. That may not be
correct in cases where we don't have gdsc at all and SMMU
will be always powered on. Use GDSC based status checkers
before accessing SMMU registers.

Change-Id: I0607bb120ffc7ed2d29615d5202a81ef11779714
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>