drm/i915/dsi: use compressed pixel format with DSC

When compression is enabled, configure the DSI transcoder to use
compressed format.

Suggested-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0e58022ce5425560b3b31062c41de385a736c8b1.1575974743.git.jani.nikula@intel.com