drm/mm: Convert drm_mm_node booleans to bitops

A straightforward conversion of assignment and checking of the boolean
state flags (allocated, scanned) into non-atomic bitops. The caller
remains responsible for all locking around the drm_mm and its nodes.

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/20191003210100.22250-4-chris@chris-wilson.co.uk