radeonsi: add a comment about early prim exports

Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35529>

src/gallium/drivers/radeonsi/si_shader.c

@@ -1109,6 +1109,8 @@ bool gfx10_ngg_export_prim_early(struct si_shader *shader)
 
    return sel->stage != MESA_SHADER_GEOMETRY &&
           !gfx10_ngg_writes_user_edgeflags(shader) &&
+          /* gfx10.x is sometimes slower with the late primitive export, so use the early prim
+           * export by default. */
           sel->screen->info.gfx_level < GFX11;
 }