drm/nv50: drop explicit yields in favour of smaller PFIFO timeslice

This gives a small, but noticeable performance gain at lower performance
levels, and unchanged at the higher ones.

With this commit, we're now using the same timeslice size as the NVIDIA
binary driver currently does, and dropping an unknown bit that NVIDIA
no longer appear to set.

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