drm/amd/powerplay: fix bug fail to remove sysfs when rmmod amdgpu.

when hw_fini, pp will disable dpm.so remove sysfs before
disable dpm.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>