drm/i915: Clean up hsw/bdw/skl/kbl buf trans funcs

Split the hsw/bdw/skl/kbl get_buf_trans() functions into
clean platform specific variants.

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/20210608073603.2408-11-ville.syrjala@linux.intel.com