drm/doc: Overview documentation for drm_mm.c

kerneldoc polish will follow in the next patch.

Hopefully documenting the lru scan support a bit better spurs someone
to give this a shot in the ttm eviction code. At least in i915 it
helped quite a lot with memory thrashing on platforms where eviction
was (we've fixed that too meanwhile) fairly expensive.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>