drm/i915: Remove pointless 'ddi_translations' local variable

skl_get_buf_trans_*() don't need the 'ddi_translations' local variable
since all they with is assign and return. Just return the right thing
directly and get rid of the local variable.

v2: Resolve conflicts due to KBL

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)