drm/amdgpu: explicitly set mmGDS_VMID0_BASE to 0

Explicitly set mmGDS_VMID0_BASE to 0. Also update
GDS_VMID0_BASE/_SIZE with direct register writes.

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