drm: Avoid NULL dereference for DRM_LEGACY debug message

smatch warns:

	drivers/gpu/drm/drm_lock.c:188 drm_legacy_lock() warn:
	variable dereferenced before check 'master->lock.hw_lock' (see line 177)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161127170910.29106-2-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_lock.c

@@ -176,7 +176,8 @@ int drm_legacy_lock(struct drm_device *dev, void *data,
 
 	DRM_DEBUG("%d (pid %d) requests lock (0x%08x), flags = 0x%08x\n",
 		  lock->context, task_pid_nr(current),
-		  master->lock.hw_lock->lock, lock->flags);
+		  master->lock.hw_lock ? master->lock.hw_lock->lock : -1,
+		  lock->flags);
 
 	add_wait_queue(&master->lock.lock_queue, &entry);
 	spin_lock_bh(&master->lock.spinlock);