drm/amd/pp: OverDrive gfx domain voltage on Tonga

Also ajust the gfx domain voltage on Tonga when user overdriver
the voltage.

For Tonga, Driver do not update user's setting to voltage table
in smu, we only pick up a minimum value from voltage table that
not less than the user's setting.

v2: fix a typo

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>