drm/amdgpu: Fix GART page table s-bit

For the new 2-level GART table, the last PDE0 points
to PTB. Since PTB is in vram and right now we are
runing under s=0 mode (vram is treated as FB carveout),
so the s bit of this PDE0 should be set to 0.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>