drm: kselftest for drm_mm and alignment

Check that we can request alignment to any power-of-two or prime using a
plain drm_mm_node_insert(), and also handle a reasonable selection of
primes.

v2: Exercise 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/20161222083641.2691-14-chris@chris-wilson.co.uk