drm/nouveau/kms/nv50: separate out base/ovly channel usage bounds commit

This commit separates the calculation of EVO state from the commit, in
order to make the same code useful for atomic modesetting.

The legacy interfaces have been wrapped on top of them.

We're no longer touching the overlay channel usage bounds as of this
commit.  The code to do so is in place for when overlay planes are
added.

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