drm/i915: correctly update crtc->x/y in set_base

While reworking the modeset sequence, this got lost in

commit 25c5b266
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Jul 8 22:08:04 2012 +0200

    drm/i915: implement new set_mode code flow

I've noticed this because some Xorg versions seem to set up a new mode
with every crtc at (0,0) and then pan to the right multi-monitor
setup. And since some hacks of mine added more calls to mode_set using
the stored crtc->x/y my multi-screen setup blew up.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>