struct drm_mm fixes

[Chris Wilson]

For a long time, we've known DRM_MM_SEARCH_HIGH/CREATE_TOP to be slow and
broken (both not returning the right hole nor the right alignment). This
series ends by fixing that but takes a quick detour through adding a set
of testcases to catch the broken behaviour and hopefully keep the working
set, and takes a stop at tightening eviction.

I've folded in most of Joonas's comments in the selftests, but didn't
apply his r-b as they have subtantially changed (mostly layout, splitting
of library functions and what-not, fundamental test logic/efficacy should
be unchanged, hopefully).
-Chris