msm: kgsl: Update calculation for graphics IOMMU context IDs

Calculate graphics IOMMU context IDs using the 'label' property
of the device tree node of that context. Context IDs are independent
of the register addresses of the SMMU device and its contexts, which
change for every target.

Change-Id: I16825d7248081997a442d4fb59ca5b04b88e33fd
Signed-off-by: Lokesh Batra <lbatra@codeaurora.org>
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>