drm/i915: add crtc->enable/disable vfuncs insted of dpms

Because that's what we're essentially calling. This is the first step
in untangling the crtc_helper induced dpms handling mess we have - at
the crtc level we only have 2 states and the magic is just in
selecting which one (and atm there isn't even much magic, but on
recent platforms where not even the crt output has more than 2 states
we could do better).

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