drm/i915: Move bxt_ddi_vswing_sequence() call into intel_ddi_pre_enable() for HDMI

drm/i915: Move bxt_ddi_vswing_sequence() call into intel_ddi_pre_enable() for HDMI

Now that the SKL iboost programming is done from intel_ddi_pre_enable()
for HDMI, let's move the BXT bxt_ddi_vswing_sequence() call there as
well. This makes things look more similar to the DP/eDP case which
is handled in ddi_signal_levels().

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468328376-6380-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>