radv: fix DCC fast clear code for intensity formats (correctly)

Previous fix was pretty bogus.

This fixes a rendering regression with Nier (minimap too large).

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1943
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1952


Fixes: ea92273c ("radv: fix DCC fast clear code for intensity formats")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit c644644c)