drm/i915/icl: Define DSI transcoder timing registers

This patch defines registers and bitfields used for
programming DSI transcoder's horizontal and vertical
timings.

v2: Remove TRANS_TIMING_SHIFT definition

v3 by Jani:
 - Group macros by transcoder

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/dcc329280e3aca5b4fc3482c5bcaa0cac043c5d8.1539613303.git.jani.nikula@intel.com