drm/i915/dp: Use intel_display instead of drm_i915_private

Use intel display instead of drm_i915_private in
mtl_ddi_prepare_link_retrain & mtl_port_buf_ctl_program
functions.

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250103051705.145161-2-suraj.kandpal@intel.com