drm/nouveau/kms/nv50-: Assert we hold nv50_disp->lock in nv50_head_flush_*

Now that we've had one bug that occurred in nouveau as the result of
nv50_head_flush_* being called without the appropriate locks, let's add
some lockdep asserts to make sure this doesn't happen in the future.

Reviewed-by: Dave Airlie <[e-mail]>
Signed-off-by: Lyude Paul <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

drivers/gpu/drm/nouveau/dispnv50/head.c

···
 	union nv50_head_atom_mask clr = {
 		.mask = asyh->clr.mask & ~(flush ? 0 : asyh->set.mask),
 	};
+
+	lockdep_assert_held(&head->disp->mutex);
+
 	if (clr.crc)  nv50_crc_atomic_clr(head);
 	if (clr.olut) head->func->olut_clr(head);
 	if (clr.core) head->func->core_clr(head);
···
 void
 nv50_head_flush_set(struct nv50_head *head, struct nv50_head_atom *asyh)
 {
+	lockdep_assert_held(&head->disp->mutex);
+
 	if (asyh->set.view   ) head->func->view    (head, asyh);
 	if (asyh->set.mode   ) head->func->mode    (head, asyh);
 	if (asyh->set.core   ) head->func->core_set(head, asyh);