drm/i915/icl: Implement voltage swing programming sequence for MG PHY DDI

This sequence is used to setup voltage swing before enabling MG PHY DDI
as well as for changing the voltage during DisplayPort Link training.

For ICL, there are two types of DDIs. This sequence needs to be used
for MG PHY DDI which is ports C-F.

v6 (From Manasi):
* Add programming for MG_CLKHUB and MG_TX_DCC as per the
spec updates

v5 (from Paulo):
* Checkpatch.
v4 (from Paulo):
* Fix bogus error message
* Fix copy+paste bugs (missing s/TX1/TX2/ after copy+paste)
* Use the new mask names
* Stay under 80 columns
* Add some blank lines
v3:
* Clear the regs before writing (Paulo)
v2:
* Rename to MG PHY in the function def (Jani Nikula)
* Rebase on top of new revision of other patches in series

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1530225344-20373-2-git-send-email-manasi.d.navare@intel.com