drm/i915/icl: Configure DSI Dual link mode

This patch configures DSI video mode dual link by
programming DSS_CTL registers.

v2: Use new bitfield definitions from Anusha's patch
    Correct register to be programmed and use max
    depth buffer value (James)

v3 by Jani:
 - checkpatch fixes

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/d8962f4e1beffc2099b8bef8cb5a5191aa1d9efd.1543500286.git.jani.nikula@intel.com