drm/nouveau/dmaobj: move parent class check to bind() method

Otherwise when nvc0- gains a bind() method (disp needs it), the fifo
engine will attempt to create a dma object for the push buffer, which
is unnecessary on fermi.

The only sane place to put these checks is in the bind method itself,
and have it unconditionally called from wherever it might be needed.

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