drm/amd/powerplay: add shared definitions for di/dt feature.

v1: delete some comflict definitions between polaris and fiji.

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>