drm/nouveau: remove remnants of nouveau_pgraph_engine from nouveau_channel

The nouveau_wait_for_idle() call should hopefully not have been actually
necessary, we *do* wait for the channel to go idle already.  If it's
an issue somehow, the chipset-specific hooks can wait for idle themselves
before taking the lock.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>