drm/nv50-nvc0: make sure vma is definitely unmapped when destroying bo

Somehow fixes a misrendering + hang at GDM startup on my NVA8...

My first guess would have been stale TLB entries laying around that a new
bo then accidentally inherits.  That doesn't make a great deal of sense
however, as when we mapped the pages for the new bo the TLBs would've
gotten flushed anyway.

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