ac,radeonsi: don't export null from PS if it has no effect on gfx10+

ac,radeonsi: don't export null from PS if it has no effect on gfx10+

We just need to pass the uses_discard flag to the epilog.

The hw skips the export anyway. This will hang if SPI registers declare
an output format or KILL_ENABLE is set because those cases require
an export with done=1.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16885>