drm/i915/ddi: Simplify the port enabling via DDI_BUF_CTL

In the past intel_digital_port::dp.prepare_link_retrain() could be
called directly (vs. from a modeset) to retrain an enabled link. In that
case the port had to be first disabled and then re-enabled. That changed
with commit 2885d283 ("drm/i915/dp: Retrain SST links via a modeset
commit"), after which the only way prepare_link_retrain() can be called
is from a modeset during link training when the port is still disabled.
Simplify things accordingly, assuming the disabled port state.

v2: Don't use drm_i915_private in intel_ddi_prepare_link_retrain(). (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250214142001.552916-6-imre.deak@intel.com