drm/gf100-/bar: don't fill in bar->alloc until after all vm setup done

gpuobj has a condition of (bar && bar->alloc) around usage to avoid
some nasty ordering issues (which, i've now been reminded to add a
todo about fixing...) between bar and vm.

The bar->alloc part of the condition isn't currently necessary (it
used to be, another change made bar always NULL where it matters),
so we got lucky.  That won't be the case for much longer.

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