drm/i915: push commit_output_state past crtc disabling

drm/i915: push commit_output_state past crtc disabling

This requires a few changes
- We still need a noop function for crtc->disable, becuase the fb
  helper is a bit too intimate with the crtc helper.
- We need to clear crtc->fb ourselves in intel_crtc_disable now that
  we no longer rely on the helper's disable_unused_functions to do
  that.
- We need to split out the sare update code, becuase the crtc code
  can't call update_dpms any more, it needs to disable the crtc
  unconditionally. This is because we now keep onto the encoder ->
  crtc mapping of the (still) active output pipe configuration.
- To check that we really disable a crtc that still has encoders,
  insert a WARN_ON(!enabled) in the crtc disable function.
- Lastly, we need to walk over all crtcs to update their enabled state
  after having called commit_output_state - for all disabled crtcs the
  crtc helper code has done that for us previously.

v2: Update connector dpms and encoder->connectors_active after
disabling the crtc, too.

v3: Noop-out intel_encoder_disable. Similarly to the crtc disable
callback used by the crtc helper code we can't simply remove all these
encoder callbacks: The fb helper (which we still use) has a rather
incetious relationship with the crtc helper code ...

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