drm/i915/icl: Define DSI panel programming registers

This patch defines DSI_CMD_RXCTL, DSI_CMD_TXCTL registers,
bitfields, masks and macros used for configuring DSI panel.

v2: Define remaining bitfields

v3 by Jani:
 - Alignment fix

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/37b41fe08ce50c3d9ef7d55c03d12a8a10a252d6.1539613303.git.jani.nikula@intel.com