drm/mm: Use helpers for drm_mm_node booleans

In preparation for rearranging the booleans into a flags field, ensure
all the current users are using the inline helpers and not directly
accessing the members.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191003210100.22250-3-chris@chris-wilson.co.uk