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

This is an important part of the DDI initalization as well as
for changing the voltage during DisplayPort link training.

The Voltage swing seqeuence is similar to Cannonlake.
However it has different register definitions and hence
it makes sense to create a separate vswing sequence and
program functions for ICL to leave room for more changes
in case the Bspec changes later and deviates from CNL sequence.

v2:
Use ~TAP3_DISABLE for enbaling that bit (Jani Nikula)

v3:
* Use dw4_scaling column for PORT_TX_DW4 values (Rodrigo)

v4:
* Call it combo_vswing, use switch statement (Paulo)

v5 (from Paulo):
* Fix a typo.
* s/rate < 600000/rate <= 600000/.
* Don't remove blank lines that should be there.

v6:
* Rebased by Rodrigo on top of Cannonlake changes
  where non vswing sequences are not aligned with iboost
  anymore.

v7: Another rebase after an upstream rework.

v8 (from Paulo):
* Adjust the code to the upstream output type changes.
* Squash the patch that moved some functions up.
* Merge both get_combo_buf_trans functions in order to simplify the
  code.
* Change the changelog format.

v9 (from Paulo):
* Use RTERM_SELECT instead of SCALING_MODE_SEL.
* Adjust the output type handling according to how the other platforms
  do it now.

v10 (from Paulo):
* Fix comment left out from v9 changes (Rodrigo).

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: James Ausmus <james.ausmus@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180328215803.13835-8-paulo.r.zanoni@intel.com