drm/i915: Do not dereference NULL crtc or fb until after checking

Fixes regression from
commit 4906557e
Author: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Date:   Thu Jul 11 18:45:05 2013 -0300

    drm/i915: Hook PSR functionality

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67526


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>