drm/amd/pp: Skip fan attributes if fan not present

With powerplay enabled, also need to skip fan attributes
if no fan present.

Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>