You'll need to set the format_mod_supported hook as well, otherwise the kernel will expose a bogus IN_FORMATS prop with one modifier and zero formats.
- [PATCH] drm/vkms: set plane modifiers José Expósito
- Re: [PATCH] drm/vkms: set plane modifiers Simon Ser
- Re: [PATCH] drm/vkms: set plane modifiers José Expósito