drm/amd/powerplay: don't succeed in getters if fan is missing

Otherwise callers end up using uninitialized data.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>