drm/i915/icl: Add DSI packet payload/header registers

This patch defines payload/header registers for each DSI
transcoder used for transmitting DSI packets.

v2 by Jani:
 - Drop full register mask and shift for payload
 - Use lower case for hex 0x

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/18275c49215e512347a14bc38715314c2d6f95a4.1540900289.git.jani.nikula@intel.com