drm/i915/ddi: Don't rewrite DDI_BUF_CTL reg during DP link training

drm/i915/ddi: Don't rewrite DDI_BUF_CTL reg during DP link training

The value we program to DDI_BUF_CTL changes at the following places:
- At enabling/disabling the output to configure the port width etc, and
  to enable/disable the DDI BUF function.
- At the beginning/end of link re-training to disable/re-enable the DDI
  BUF function.
- On HSW/BDW/SKL to change the voltage swing/pre-emph levels.

Except of the above the value we program to the DDI_BUF_CTL register
(intel_dp->DP) doesn't change, so no need to reprogram the register when
changing the link training patterns (which is programmed via the
DP_TP_CTL register on DDI platforms).

v2:
- Fix the commit message wrt. voltage/pre-emph level values in
  intel_dp->DP. (Ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200714153141.10280-2-imre.deak@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>