drm/msm/gpu: add per-process pagetables param

For now it always returns '0' (false), but once the iommu work is in
place to enable per-process pagetables we can update the value returned.

Userspace needs to know this to make an informed decision about exposing
KHR_robustness.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>