drm/i915; Return the whole buf_trans struct from get_buf_trans()

Raise the abstraction level of the get_buf_trans() functions
a bit more by returning the whole wrapper intel_ddi_buf_trans
struct.

v2: Handle adl-p

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210608073603.2408-8-ville.syrjala@linux.intel.com