Reinstate "drm/probe-helpers: Drop locking from poll_enable""

This reverts commit 54a07c7b,
and reinstates the original.

[airlied: this might be a bad plan for git].

commit 3846fd9b
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Jan 11 10:01:17 2017 +0100

    drm/probe-helpers: Drop locking from poll_enable

    It was only needed to protect the connector_list walking, see

    commit 8c4ccc4a
    Author: Daniel Vetter <daniel.vetter@ffwll.ch>
    Date:   Thu Jul 9 23:44:26 2015 +0200

        drm/probe-helper: Grab mode_config.mutex in poll_init/enable

    Unfortunately the commit message of that patch fails to mention that
    the new locking check was for the connector_list.

    But that requirement disappeared in

    commit c36a3254
    Author: Daniel Vetter <daniel.vetter@ffwll.ch>
    Date:   Thu Dec 15 16:58:43 2016 +0100

        drm: Convert all helpers...