drm/i915: Set mode_changed for audio in intel_modeset_pipe_config()

A follow up patch will make intel_modeset_compute_config() deal with
multiple crtcs, so move crtc specific stuff into the lower level crtc
specific function.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
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>