drm/mm: Use clear_bit_unlock() for releasing the drm_mm_node()

A few callers need to serialise the destruction of their drm_mm_node and
ensure it is removed from the drm_mm before freeing. However, to be
completely sure that any access from another thread is complete before
we free the struct, we require the RELEASE semantics of
clear_bit_unlock().

This allows the conditional locking such as

Thread A			Thread B
  mutex_lock(mm_lock);		  if (drm_mm_node_allocated(node)) {
  drm_mm_node_remove(node);	    mutex_lock(mm_lock);
  mutex_unlock(mm_lock);	    if (drm_mm_node_allocated(node))
				      drm_mm_node_remove(node);
				    mutex_unlock(mm_lock);
				  }
				  kfree(node);

to serialise correctly without any lingering accesses from A to the
freed node. Allocation / insertion of the node is assumed never to race
with removal or eviction scanning.

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/20191003210100.22250-5-chris@chris-wilson.co.uk