drm/amdgpu: add amdgpu_gmc_pd_addr helper

Add a helper to get the root PD address and remove the workarounds from
the GMC9 code for that.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>