drm/amd/pm: correct the checks for fan attributes support

On functionality unsupported, -EOPNOTSUPP will be returned. And we rely
on that to determine the fan attributes support.

Fixes: 79c65f3f ("drm/amd/pm: do not expose power implementation details to amdgpu_pm.c")

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>