Weston does not use scanout for weston-simple-dmabuf-feedback on AMD Gen >= 9

Weston doesn't use scanout/overlay planes when running weston-simple-dmabuf-feedback on my AMD system (with explicit modifier support).

From the default feedback it will pick the modifier 0x20000001896bb03 which is apparently not scanout compatible.Other compositors will send a scanout tranche and weston-simple-dmabuf-feedback will switch to 0x200000018967b03, allowing successful scanout.

Here are two logs for Weston with --logger-scopes=log,drm-backend:

• weston-amd-default-new.log default, no occurrence of "Using plane-only state composition"
• weston-amd-force-0x200000018937b03-new.log force 0x200000018967b03 modifier -> successful scanout, lots of occurrences of "Using plane-only state composition"