radv: fix DCC fast clear code for intensity formats

This fixes a rendering issue with DiRT 4 on GFX10. Only GFX10 was
affected because intensity formats are different.

Cc: 19.2 <mesa-stable@lists.freedesktop.org>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1923


Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit ea92273c)