drm/i915: Don't modeset with old mode when set_crtc fails

The modeset code is now properly divided in two phases, so that it only
changes hardware state if it succeeds, so there's no ill-effect that
needs to be undone on failure anymore.

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>