drm/nouveau/kms/nv50-: atom: fix an incorrect NULL check on list iterator

stable inclusion
from stable-v5.10.121
commit addf0ae792589f33b2d7daabd53e30fc5bc33780
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5L6CQ

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=addf0ae792589f33b2d7daabd53e30fc5bc33780



--------------------------------

commit 6ce4431c upstream.

The bug is here:
	return encoder;

The list iterator value 'encoder' will *always* be set and non-NULL
by drm_for_each_encoder_mask(), so it is incorrect to assume that the
iterator value will be NULL if the list is empty or no element found.
Otherwise it will bypass some NULL checks and lead to invalid memory
access passing the check.

To fix this bug, just return 'encoder' when found, otherwise return
NULL.

Cc: stable@vger.kernel.org
Fixes: 12885ecb ("drm/nouveau/kms/nvd9-: Add CRC support")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
[Changed commit title]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220327073925.11121-1-xiam0nd.tong@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>