drm/i915/icl: Set max return packet size for DSI panel

This patch programs maximum size of the payload transmitted
from peripheral back to the host processor using short packet
as a part of panel programming.

v2: Rebase

v3 by Jani:
 - Add FIXME note.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ed9df910326adf32eb2bc1cd1a5097d0dda94da8.1540900289.git.jani.nikula@intel.com