drm: kselftest for drm_mm and eviction

Check that we add arbitrary blocks to the eviction scanner in order to
find the first minimal hole that matches our request.

v2: Refactor out some common eviction code for later

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222083641.2691-15-chris@chris-wilson.co.uk