drm/i915: Hook up pfit for DSI

Add the scaling mode property to DSI connectors, handle changes in the
property value, and compute the panel fitter state during
.compute_config().

v2: Handle BXT as well

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/1460488478-18311-5-git-send-email-ville.syrjala@linux.intel.com


Tested-by: Jani Nikula <jani.nikula@intel.com>