drm/i915: Call intel_ddi_init_dp_buf_reg() earlier

I want intel_dp->DP to be fully populated by the time the
initial vswing programming happens. To that end move the
intel_ddi_init_dp_buf_reg() call to an earlier spot.

Additionally we don't want intel_ddi_init_dp_buf_reg() to
set DDI_BUF_CTL_ENABLE since the port should only get enabled
at the start of link training (see intel_ddi_prepare_link_retrain()).
So any earlier write to the register should not set the enable bit.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210930134310.31669-2-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>