tu: Move UBWC layout into fdl6_layout() and use that function.

tu: Move UBWC layout into fdl6_layout() and use that function.

This gets us shared non-UBWC layout code between gallium and turnip.
Until I fix up the rest of gallium to handle UBWC mipmapping, we do the
single-level UBWC setup in gallium as a fixup after layout.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>