drm/amd/powerplay: fix a couple locking issues

We should return unlock on the error path in pp_dpm_dispatch_tasks()
and there is a double lock bug in pp_dpm_set_sclk_od().

Fixes: 2a507105

 ("drm/amd/powerplay: add global PowerPlay mutex.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>