(pre-stable) drm/i915: add PANEL_UNLOCK_REGS definition

BugLink: http://launchpad.net/bugs/578673
BugLink: http://launchpad.net/bugs/561802

In some cases, unlocking the panel regs is safe and can help us avoid a
flickery, full mode set sequence.  So define the unlock key and use it.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 4a655f04

 upstream)
Signed-off-by: Manoj Iyer <manoj.iyer@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Acked-by: Steve Conklin <sconklin@canonical.com>
Signed-off-by: Steve Conklin <sconklin@canonical.com>