drm/i915/sdvo: Use pipe_config->limited_color_range consistently

We in the pre_enable hook we should only rely on the pipe config and
not on some other state set through properties or detect functions.

Reviewed-by: Naresh Kumar Kachhi <naresh.kumar.kachhi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>