drm/i915: Reject 'Center' scaling mode for eDP/DSI on GMCH platforms

drm/i915: Reject 'Center' scaling mode for eDP/DSI on GMCH platforms

We don't have a LVDS_BORDER_ENABLE type of bit for either eDP or DSI,
and just trying to frob the display timings to include borders results
in a corrupted picture. So reject the 'Center' scaling mode on GMCH
platforms for eDP and DSI.

TODO: Should really filter out the unsupported modes from the prop,
but that would be fairly invasive since the prop is now created and
stored by drm core. So leave it for a rainy day.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460488478-18311-6-git-send-email-ville.syrjala@linux.intel.com
Tested-by: Jani Nikula <jani.nikula@intel.com>