zink: use real A8_UNORM when possible

zink: use real A8_UNORM when possible

this is tricky because drivers are exposing their native support, but
that support may not fit the specific needs of the format

as such, (almost) every use of format where alpha emulation workarounds are
present now need to add a second layer of workarounds in order to handle
the case of possibly-genuine A8 (which might also be emulated A8 even if
the driver supports A8 for some things)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24685>