drm/i915: Disallow interlaced modes on g4x DP outputs

Looks like interlaced DP output doesn't work on g4x either. Not all
that surprising considering we already established that interlaced
DP output is busted on VLV/CHV.

Cc: stable@vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180613160553.11664-1-ville.syrjala@linux.intel.com