drm/nv20: Don't use pushbuf calls on the original nv20.

The "return" command is buggy on the original nv20, it jumps back to
the caller address as expected, but it doesn't clear the subroutine
active bit making the subsequent pushbuf calls fail with a "stack"
overflow.

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