drm/nouveau: prepare for enabling svm with existing userspace interfaces

For a channel to make use of SVM features, it requires a different GPU MMU
configuration than we would normally use, which is not desirable to switch
to unless a client is actively going to use SVM.

In order to supporting SVM without more extensive changes to the userspace
interfaces, the SVM_INIT ioctl needs to replace the previous configuration
safely.

The only way we can currently do this safely, accounting for some unlikely
failure conditions, is to allocate the new VMM without destroying the last
one, and prioritising the SVM-enabled configuration in the code that cares.

This will get cleaned up again further down the track.

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