drm/amdgpu: Add function to allocate and fill PDB0

Add functions to allocate PDB0, map it for CPU access,
and fill it.

Those functions are only used for 2-level vmid0 page
table construction

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>