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>

src/amd/llvm/ac_llvm_build.c

@@ -1996,10 +1996,16 @@ void ac_build_export(struct ac_llvm_context *ctx, struct ac_export_args *a)
    }
 }
 
-void ac_build_export_null(struct ac_llvm_context *ctx)
+void ac_build_export_null(struct ac_llvm_context *ctx, bool uses_discard)
 {
    struct ac_export_args args;
 
+   /* Gfx10+ doesn't need to export anything if we don't need to export the EXEC mask
+    * for discard.
+    */
+   if (ctx->gfx_level >= GFX10 && !uses_discard)
+      return;
+
    args.enabled_channels = 0x0; /* enabled channels */
    args.valid_mask = 1;         /* whether the EXEC mask is valid */
    args.done = 1;               /* DONE bit */