drm/i915: Clean up jsl/ehl buf trans functions

The jsl/ehl buf trans functions are needlessly complicated.
Simplify them.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210608073603.2408-16-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>