drm/amd/powerplay: unlock on error in smu_resume()

This function needs to drop the mutex before returning.

Fixes: f7e3a577 ("drm/amd/powerplay: check SMU engine readiness before proceeding on S3 resume")
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>