broadcom/compiler: Set up passthrough Z when doing FS discards.

In order to keep early-Z from writing early in a discard shader, you need
to set the "modifies Z" bit in the shader state (which the new
prog_data.discards will indicate).  Then, in the shader we do a TLB write
to make Z passthrough happen (the QPU result is ignored, so we use a NULL
source).

src/broadcom/compiler/vir.c

@@ -762,8 +762,9 @@ uint64_t *v3d_compile_fs(const struct v3d_compiler *compiler,
 
         v3d_set_prog_data(c, &prog_data->base);
         v3d_set_fs_prog_data_inputs(c, prog_data);
-        if (c->s->info.outputs_written & (1 << FRAG_RESULT_DEPTH))
-                prog_data->writes_z = true;
+        prog_data->writes_z = (c->s->info.outputs_written &
+                               (1 << FRAG_RESULT_DEPTH));
+        prog_data->discard = c->s->info.fs.uses_discard;
 
         return v3d_return_qpu_insts(c, final_assembly_size);
 }