[Intel-gfx] drm_mm range manager fixes, take 2

[Chris Wilson]

The goal of this series is to fix top-down allocations to be allocated
from the top and aligned correctly, introduce bottom-up allocations, and
speed up searches and tighten evictions.

More polish on the test cases to reduce code duplication and to improve
expectation checking. And a little more polish on the patches leading up
to the fixes.
-Chris