drm: kselftest for drm_mm and top-down allocation

Check that if we request top-down allocation from drm_mm_insert_node()
we receive the next available hole from the top.

v2: Flip sign on conditional assert.

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/20161222083641.2691-17-chris@chris-wilson.co.uk