drm/panfrost: Remove unused id_mask from struct panfrost_model

The id_mask field of struct panfrost_model has never been used.

Fixes: f3ba9122 ("drm/panfrost: Add initial panfrost driver")
Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241025140008.385081-1-steven.price@arm.com