drm/i915: Remove save/restore logic from intel_crtc_set_config()

This is no longer necessary since we only update the staged config on
successfull modeset. The new configuration is stored in an atomic state
struct which is freed in case of failure.

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>