drm/i915: warn about missing ->get_buf_trans initialization

Make sure each DDI platform has sane ->get_buf_trans initialized.

Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220503082134.4128355-1-jani.nikula@intel.com