drm/i915: Enable voltage swing before enabling DDI_BUF_CTL.

Sequences for DisplayPort asks us to
" Configure voltage swing and related IO settings.
Refer to DDI Buffer section."

before "Configure and enable DDI_BUF_CTL"

On BXT and CNL this means to execute the ddi vswing sequences.

At this point these sequences calls are getting duplicated for DP
because they are all called from DP link trainning sequences.

However this patch is not yet removing it before a futher discussion
since spec also allows that during link training without disabling
anything:

"
Notes
Changing voltage swing during link training:
Change the swing setting following the DDI Buffer section.
The port does not need to be disabled.
"

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170829232230.23051-4-rodrigo.vivi@intel.com

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