BugLink: http://bugs.launchpad.net/bugs/1540390

Part of upstream commit:

commit cacf9f7359a8f1347f47b0905cc6fbb4382ac347
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Dec 8 11:55:07 2015 +0000

    drm/i915: Add soft-pinning API for execbuffer
Signed-off-by: Timo Aaltonen <timo.aaltonen@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>