nil/modifiers: Clarify drm_format_mods_for_format rejecting modifiers for unsupported color formats

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Reviewed-by: Mel Henning <mhenning@darkrefraction.com>
Tested-by: Mary Guillemard <mary@mary.zone>

Backport-to: 26.1
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40473>
(cherry picked from commit 7f5767a2f2)

.pick_status.json

@@ -2214,7 +2214,7 @@
         "description": "nil/modifiers: Clarify drm_format_mods_for_format rejecting modifiers for unsupported color formats",
         "nominated": true,
         "nomination_type": 4,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null

src/nouveau/nil/modifiers.rs

@@ -268,6 +268,7 @@ pub fn drm_format_mods_for_format(
         return;
     }
 
+    // This rejects unsupported color formats like YCbCr and any others
     if !format.supports_color_targets(dev) {
         return;
     }