panfrost: Set overdraw_alpha[01] flags

panfrost: Set overdraw_alpha[01] flags

These basically correspond to the alpha_zero_nop and alpha_one_store flags we
already compute and set. Except those flags don't exist on Valhall, so these
need to be used instead (on Bifrost, in addition .. unclear why the duplication
on Bifrost).

Set these flags when we can. Ostensibly this is for performance (neglible
improvement on glmark2 score), but mostly I want to get us using the hardware
optimally.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16748>