drm/nv50: Fix large 3D performance regression caused by the interchannel sync patches.

Reported-by: Christoph Bumiller <e0425955@student.tuwien.ac.at>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Tested-by: Maarten Maathuis <madman2003@gmail.com>
Tested-by: Xavier Chantry <chantry.xavier@gmail.com>
Tested-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>