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

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