drm/i915: Use drm_atomic_helper_swap_state in intel_atomic_commit.

And update crtc->config to point to the new state. There is no point
in swapping only part of the state when the rest of the state
should be untouched.

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>