i965/fs: Pass a BAD_FILE header source to LOAD_PAYLOAD in emit_texture_gen7().

So that it's left uninitialized by LOAD_PAYLOAD, we only need to
reserve space for it in the message since it will be initialized
implicitly by the generator.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_fs_visitor.cpp

@@ -483,7 +483,7 @@ fs_visitor::emit_texture_gen7(ir_texture_opcode op, fs_reg dst,
        * need to offset the Sampler State Pointer in the header.
        */
       header_size = 1;
-      sources[0] = fs_reg(GRF, alloc.allocate(1), BRW_REGISTER_TYPE_UD);
+      sources[0] = fs_reg();
       length++;
    }