drm/amd/powerplay: delete flag PP_VALID

don't need to check pp_valid, all pp
export functions are moved to ip_funcs
and pp_funcs. so just need to check the
function point.

Reviewed-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>