drm/nouveau: set TASK_(UN)INTERRUPTIBLE before schedule_timeout()

set_current_state() is called only once before the first iteration.
After return from schedule_timeout() current state is TASK_RUNNING. If
we are going to wait again, set_current_state() must be called.

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>