drm/i915: Pass new_config down do crtc_compute_clock

This reduces the number of direct users of crtc->new_config, opening up
the possibilty of removing it altogether.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>