drm/amdgpu/pm: return an error during GPU reset or suspend (v2)

Return an error for sysfs and debugfs power interfaces during
gpu reset and suspend.  Prevents access to the hw while it may
be in an unusable state.

v2: squash in fix to drop suspend check

Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>