drm: Skip initialising the drm_mm_node->hole_stack

As we always add this to the drm_mm->hole_stack as our first operation,
we do not need to initialise the list node.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1470236651-678-3-git-send-email-chris@chris-wilson.co.uk