drm/i915: Swap atomic state in legacy modeset

Replace the commit output state function with a simple swap of states.
Note that we still need to reconcile the legacy state after the swap,
since there are still code that relies on those.

Also note that even though changes to the state of a crtc different than
the one passed as an argument to __intel_set_mode() will be saved, the
modeset logic still deals with only one crtc.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>