drm/i915: Don't use staged config for VLV cdclk calculations

Now that we use a drm atomic state for the legacy modeset, it is
possible to get rid of the usage of intel_crtc->new_config in the
function intel_mode_max_pixclk().

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