panfrost: Fix background showing when using discard
This fixes problems in a number of games, including SuperTuxKart, OpenMW and RVGL. v2: Use MALI_READS_ZS | 0x20 instead of MALI_WRITES_Z to match with the blob. Keep using 0x400 | 0x20 when depth is disabled. Closes: #2620 Reviewed-by:Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com> (cherry picked from commit 0dd24b38) Part-of: <mesa/mesa!5193>