msm: kgsl: Avoid calling get_secure_bus for iommu_domain_alloc for 8996

For 8994, we needed to call iommu_domain_alloc() with
msm_iommu_sec_bus_type as a parameter. The ARM SMMU driver does
not distinguish between the secure and non secure domains using
this parameter. Therefore, continue using the platform_bus_type
as the parameter to iommu_domain_alloc().

Change-Id: I7ccac6c5a6a4c66c277400baac0d036b68d4c7fb
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>