mesa,radeonsi: add comments about vertex and mesh pipeline shader states

They are exclusive in mesa state tracker currently, so add some comments
and assertions for developers.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37505>

src/gallium/drivers/radeonsi/si_state_draw.cpp

@@ -2333,6 +2333,14 @@ static void si_draw(struct pipe_context *ctx,
       }
    }
 
+   /* Make sure that the MS is not set when vertex pipeline is used.
+    *
+    * This is ensured by mesa state tracker to always update all graphics shader
+    * stages in any pipeline's draw call, and set the other pipeline's shader stages
+    * to be NULL when internal draw operations.
+    */
+   assert(!sctx->ms_shader_state.cso);
+
    if (unlikely(sctx->dirty_shaders_mask)) {
       if (unlikely(!(si_update_shaders<GFX_VERSION, HAS_TESS, HAS_GS, NGG>(sctx)))) {
          DRAW_CLEANUP;

src/mesa/state_tracker/st_atom.h

@@ -134,6 +134,13 @@ typedef BITSET_DECLARE(st_state_bitset, ST_NUM_ATOMS);
    ST_SET_STATE4(bitset, ST_NEW_##shader##_UBOS, ST_NEW_##shader##_ATOMICS, \
                  ST_NEW_##shader##_SSBOS, ST_NEW_##shader##_IMAGES);
 
+/* This is in fact states for vertex pipeline (as opposite to mesh pipeline).
+ * But we still include mesh shader states to ensure mesh shader is not set
+ * when using vertex pipeline (so does the mesh pipeline) to simplify driver
+ * implementation.
+ *
+ * TODO: remove mesh shader states from this macro once driver is ready.
+ */
 #define ST_PIPELINE_RENDER_STATE_MASK(bitset) \
    st_state_bitset bitset = {0}; \
    ST_SHADER_STATE_MASK(bitset, CS); \