drm/i915: get rid of intel_crtc_disable and related code, v3

Now that the dpll updates are (mostly) atomic, the .off() code is a noop,
and intel_crtc_disable does mostly the same as intel_modeset_update_state.

Move all logic for connectors_active and setting dpms to that function.

Changes since v1:
- Move drm_atomic_helper_swap_state up.
Changes since v2:
- Split out intel_put_shared_dpll removal.
Changes since v3:
- Rebase on top of latest drm-intel.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>