drm/i915: Extract mode_changed computation out of stage_output_config()

drm/i915: Extract mode_changed computation out of stage_output_config()

This should make the conversion to atomic easier, by splitting the
initialization of the atomic state from the logic that decides if a
modeset is needed.

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>