drm/amdgpu: remove DRM_AMD_POWERPLAY

Powerplay is no longer optional after the recently cleanups

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>