drm/i915: Move double wide mode handling into pipe_config

Determine the need for double wide mode already in compute_config
stage as we need that information to figure out if horizontal
coordinates need to be adjusted.

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