msm: kgsl: Do not attach secure PT if already attached

Add a check to see if the secure PT has already been attached and
do not try to keep attaching it over and over during
kgsl_iommu_start().

Change-Id: If9b4cb2408890d1d19c2a4f39f7ce10db1c86286
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>