drm/i915: Use atomic state for calculating DVO_2X_MODE on i830.

This is a small behavioral change because it leaves DVO_2X_MODE
set between crtc_disable and crtc_enable. This is probably harmless
though and if not should be fixed by calculating 2x mode before
enable/disable pll.

This is needed because intel_crtc->active will be removed eventually.

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>