drm/amd: add structures for display/powerplay interface

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Tony Cheng <tony.cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>