drm/nouveau/svm: remove useless SVM range check

When nouveau processes GPU faults, it checks to see if the fault address
falls within the "unmanaged" range which is reserved for fixed allocations
instead of addresses chosen by the core mm code. If start is greater than
or equal to svmm->unmanaged.limit, then limit will also be greater than
svmm->unmanaged.limit which is greater than svmm->unmanaged.start and the
start = max_t(u64, start, svmm->unmanaged.limit) will change nothing.
Just remove the useless lines of code.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>