drm/amdgpu: use pd addr based on gart level page table

With a recent gart page table re-construction, the gart page
table is now 2-level for some ASICs: PDB0->PTB.
In the case of 2-level gart page table, the page_table_base
of vmid0 should point to PDB0 instead of PTB.

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