drm/nouveau/imem/nv50: support eviction of BAR2 mappings

A good deal of the structures we map into here aren't accessed very often
at all, and Fedora 26 has exposed an issue where after creating a heap of
channels, BAR2 space would run out, and we'd need to make use of the slow
path while accessing important structures like page tables.

This implements an LRU on BAR2 space, which allows eviction of mappings
that aren't currently needed, to make space for other objects.

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