drm/i915: Stage new modeset state straight into atomic state

The logic that stages the state before the modeset was still updating
first the old staged config and then populating the atomic state based
on that. Change this to use only the atomic state.

Note that now the staged config is updated in the function
intel_modeset_commit_output_state(). This is done so that the modeset
check and the force restore path in the hw state read out code continue
to work.

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>