radv: fix FMASK expand with SRGB formats

Found while working on DCC for MSAA.

Fixes: 6b976024 ("radv: add support for FMASK expand")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit a7f75377)