drm/i915: clean up encoder_prepare/commit

We no longer need them. And now that all encoders are converted, we
can finally move the cpt modeset check to the right place - at the end
of the crtc_enable function.

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