msm: kgsl: Add a mechanism to enable secured MMU

MSM8996 has a different probe mechanism for the kgsl iommu node
since that node now exists in the GPU devicetree and is managed by
us. Therefore, enable the secured MMU if the secure context is
found while the DT is being parsed.

Change-Id: I037c2f78cc780e7e53dca0b31b9583355e46ee6c
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>