radeonsi: clarify the conditions when FLUSH_AND_INV_DB is needed

radeonsi: clarify the conditions when FLUSH_AND_INV_DB is needed

FLUSH_AND_INV_DB should be done when we're changing surface state
registers of a bound depth target.

When depth_clear_value changes, si_state will modify
S_028038_ZRANGE_PRECISION so we need to flush the DB caches.

Verified with the captures from bugs cited below.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1283
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1330
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4263>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4263>