drm/i915: Add double_wide readout and checking

Read the double wide pipe information from hardware in
i9xx_get_pipe_config(), and check it in intel_pipe_config_compare()

For gen4+ double_wide is always false so the comparison can be done
on all platforms.

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