drm/amdgpu: add header kgd_pp_interface.h

move powerplay and amdgpu shared structures
and definitions to kgd_pp_interface.h.  This
is the interface between the base driver
and powerplay.

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>