drm/amdgpu: fetch default VDDC curve voltages (v2)

Ask the SMU for the default VDDC curve voltage values.  This
properly reports the VDDC values in the OD interface.

v2: only update if the original values are 0

Bug: https://gitlab.freedesktop.org/drm/amd/issues/1020
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 5.5.x