drm/amdgpu: Use different gart table parameters for 2-level gart table

If use gart for FB translation, we will squeeze vram into
sysvm aperture. This requires 2 level gart table. Add
page table depth and page table block size parameters
to gmc. This is prepare work to 2-level gart table
construction

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Christian Konig <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>