drm: kselftest for drm_mm and bottom-up allocation

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

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/20170202114434.3060-2-chris@chris-wilson.co.uk