drm/i915: Prepare to track new pipe config per pipe

Add a new_config pointer to intel_crtc which will point to the new pipe
config for said crtc while intel_crtc.config will still contain the old
config during first parts of the modeset operation. This is a step
towards having the entire new state available during the compute phase,
so that we can make accurate decisions about global resource usage.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>