drm/amd/powerplay: add a new register define for APU in VI.

the ixcurrent_pg_status addr is different between APU and DGPU.

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