msm: kgsl: Hardcode pt_base value instead of using PAGE_OFFSET

The PAGE_OFFSET macro value provided by the kernel differs for
arm64 and is far beyond the range of GPU virtual addresses. Hence,
create a macro for the per process pt_base value which is
applicable to our code.

Change-Id: Ia60c8f4d26067faa26f305e6a5563f0bb7dc4962
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>