intel/compiler: Make thread_payload struct abstract

Each shader stage has its own struct and will instantiate it, so the
base class doesn't need to be instantiated anymore.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18176>

src/intel/compiler/brw_fs_visitor.cpp

@@ -1241,7 +1241,7 @@ fs_visitor::init()
    this->nir_ssa_values = NULL;
    this->nir_system_values = NULL;
 
-   this->payload_ = new thread_payload();
+   this->payload_ = NULL;
    this->source_depth_to_render_target = false;
    this->runtime_check_aads_emit = false;
    this->first_non_payload_grf = 0;