drm/amd/pm: refine amdgpu pm sysfs node error code

v1:
Returns different error codes based on the scenario to help the user app understand
the AMDGPU device status when an exception occurs.

v2:
change -NODEV to -EBUSY.

Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>