drm/amdgpu/pm: handle fan properly when card is powered down

Return an error if the user tried to check or set the fan
parameters while the card is powered down (e.g., on a PX/HG
system for example).  This makes the fan consistent with the
temperature stuff.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>