radeonsi: add drirc option "radeonsi_assume_no_z_fights"

radeonsi: add drirc option "radeonsi_assume_no_z_fights"

This option enables a performance optimization where typical non-blending
draws with depth buffer may be rasterized out-of-order (on VI+, multi-SE
chips).

This optimization can lead to incorrect results when an applications
renders multiple objects with the same Z value at the same pixel, so we
will never enable it by default. But there may be applications that could
benefit from white-listing.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>