drm/amd/powerplay: use pp_endian.h for Tonga

Drop local versions of these macros.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>