drm/nouveau: fix multiple instances of reference count leaks

On calling pm_runtime_get_sync() the reference count of the device
is incremented. In case of failure, decrement the
ref count before returning the error.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>