isl: Add some asserts about multisampled surfaces

This isn't really necessary because the API doesn't allow MSAA and
mipmapping at the same time but people forget that pretty often so it's
good to have it as documentation if nothing else.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14129>

src/intel/isl/isl_surface_state.c

@@ -439,6 +439,19 @@ isl_genX(surf_fill_state_s)(const struct isl_device *dev, void *state,
    s.CubeFaceEnableNegativeX = 1;
 
 #if GFX_VER >= 6
+   /* From the Broadwell PRM for "Number of Multisamples":
+    *
+    *    "If this field is any value other than MULTISAMPLECOUNT_1, Surface
+    *    Min LOD, Mip Count / LOD, and Resource Min LOD must be set to zero."
+    *
+    * This is fine because no 3D API allows multisampling and mipmapping at
+    * the same time.
+    */
+   if (info->surf->samples > 1) {
+      assert(info->view->min_lod_clamp == 0);
+      assert(info->view->base_level == 0);
+      assert(info->view->levels == 1);
+   }
    s.NumberofMultisamples = ffs(info->surf->samples) - 1;
 #if GFX_VER >= 7
    s.MultisampledSurfaceStorageFormat =