anv: setup scratch space correctly for RT shaders

Things are a bit confusing because we use the term "scratch" for 2
different things :
  * the buffer for register allocation spilling
  * the buffer for storing live values between splitted shaders around shader calls

Here we're fixing the missing register allocation spilling buffer.

v2: update comments (Caio)
    fix scratch bo size computation with pipeline libraries (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16970>

src/intel/vulkan/anv_private.h

@@ -3016,6 +3016,9 @@ struct anv_ray_tracing_pipeline {
     * client has requested a dynamic stack size.
     */
    uint32_t                                     stack_size;
+
+   /* Maximum scratch size for all shaders in this pipeline. */
+   uint32_t                                     scratch_size;
 };
 
 #define ANV_DECL_PIPELINE_DOWNCAST(pipe_type, pipe_enum)             \