drm/i915: Introduce hsw_get_buf_trans()

All the other platforms handle the output_type stuff in their
*_get_buf_trans() functions. Do the same for hsw/bdw/skl.

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/20210608073603.2408-3-ville.syrjala@linux.intel.com