drm/panthor: Add PANTHOR_GROUP_PRIORITY_REALTIME group priority

This adds a new value to drm_panthor_group_priority exposing the
realtime priority to userspace.

This is required to implement NV_context_priority_realtime in Mesa.

v2:
- Add Steven Price r-b

v3:
- Add Boris Brezillon r-b

Signed-off-by: Mary Guillemard <mary.guillemard@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240909064820.34982-3-mary.guillemard@collabora.com