drm/i915/icl: DSI vswing programming sequence

This patch setup voltage swing before enabling
combo PHY DDI (shared with DSI).
Note that DSI voltage swing programming is for
high speed data buffers. HW automatically handles
the voltage swing for the low power data buffers.

v2: Rebase
v3: Address various review comments related to VSWING
    programming (Jani N)

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/1537095223-5184-3-git-send-email-madhav.chauhan@intel.com