drm: readd drm_lock_free in drm_unlock

I've accidently killed a little bit too much in

commit 1da3f87e
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Aug 23 22:53:24 2010 +0200

    drm: kill kernel_context_switch callbacks

Note to self: Next time also test with AIGLX disabled.

Reported-and-Tested-by: Andy Furniss <lists@andyfurniss.entadsl.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30374


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>