drm/i915: Use helpers for drm_mm_node booleans

A subset of 71724f70 ("drm/mm: Use helpers for drm_mm_node booleans")
in order to prepare drm-intel-next-queued for subsequent patches before
we can backmerge 71724f70

 itself.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004142226.13711-1-chris@chris-wilson.co.uk