drm/i915: Remove DP_PORT_EN stuff from link training code

Setting DP_PORT_EN in intel_dp->DP is already handled by
intel_dp_enable_port() so there is no point in setting it also
from the link training code.

For DDI platforms a bit with that name doesn't even exist. The
counterpart is DDI_BUF_CTL_ENABLE, which is already set up by
intel_ddi_prepare_link_retrain(). Fortunately it is the same bit
so there was no harm in doing this from the platform independent
code as well. But it's just confusing when platform independent
code sets platform specific bits in intel_dp->DP. Just get rid
of it.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210930134310.31669-3-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak.intel.com>