drm/i915: move output commit and crtc disabling into set_mode

It's rather pointless to compute crtc->enabled twice right away ;-)

The only thing we really have to be careful about is that we frob the
dpms state only after a successful modeset and when we've actually
haven't just disabled the crtc.

Hooray for convoluted interfaces ...

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