drm: Fix locking for sysfs dpms file

With atomic drivers we need to make sure that (at least in general)
property reads hold the right locks. But the legacy dpms property is
special and can be read locklessly. Since userspace loves to just
randomly look at that all the time (like with "status") do that.

To make it clear that we play tricks use the READ_ONCE compiler
barrier (and also for paranoia).

Note that there's not really anything bad going on since even with the
new atomic paths we eventually end up not chasing any pointers (and
hence possibly freed memory and other fun stuff). The locking WARNING
has been added in

commit 88a48e29
Author: Rob Clark <robdclark@gmail.com>
Date:   Thu Dec 18 16:01:50 2014 -0500

    drm: add atomic properties

but since drivers are converting not everyone will have seen this from
the start.

Jens reported this and submitted a patch to just grab the
mode_config.connection_mutex, but we can do a bit better.

v2: Remove unused variables I failed to git add for real.

Reference: http://mid.gmane.org/20150928194822.GA3930@kernel.dk


Reported-by: Jens Axboe <axboe@fb.com>
Tested-by: Jens Axboe <axboe@fb.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>