drm/i915: s/ddi_translations/trans/

"ddi_translations" is a bit too long, let's shorten it to just "trans".

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210927182455.27119-2-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>