drm/i915: Wrap the platform specific buf trans structs into a union

In order to abstact the buf trans stuff let's wrap the platform
specific structs into a union.

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/20210608073603.2408-4-ville.syrjala@linux.intel.com