drm: kselftest for drm_mm_insert_node()

Exercise drm_mm_insert_node(), check that we can't overfill a range and
that the lists are correct after reserving/removing.

v2: Extract helpers for the repeated tests
v3: Iterate over all allocation flags

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/20161222083641.2691-11-chris@chris-wilson.co.uk