drm/i915: Hold CRTC lock whilst freezing the planes

Daniel keeps on ramping up the warning level of the DRM and our display
core to make it complain whenever the locking rules are not followed.
This caught

commit 24576d23
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Tue Mar 26 09:25:45 2013 -0700

    drm/i915: enable VT switchless resume v3

introducing an unlocked access to the CRTC whilst disabling it for
suspend.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=78114


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>