radeonsi/gfx8: use the proper dcc clear size
dcc_fast_clear_size is assigned using addrlib's dccFastClearSize, which is computed using the whole surface size (including layers) so we don't need to multiply dcc_fast_clear_size by num_layers. Closes: mesa/mesa#4530 Cc: mesa-stable Reviewed-by:Marek Olšák <marek.olsak@amd.com> Part-of: <mesa/mesa!14409> (cherry picked from commit d84e0096)