amend! drm/panthor: Add the driver frontend block

drm/panthor: Add the driver frontend block

This is the last piece missing to expose the driver to the outside
world.

This is basically a wrapper between the ioctls and the other logical
blocks.

v3:
- Use a linked list rather than xarray for list of signals.
- Simplify panthor_get_uobj_array by returning the newly allocated
  array.
- Drop the "DOC" for job submission helpers and move the relevant
  comments to panthor_ioctl_group_submit().
- Add helpers sync_op_is_signal()/sync_op_is_wait().
- Simplify return type of panthor_submit_ctx_add_sync_signal() and
  panthor_submit_ctx_get_sync_signal().
- Drop WARN_ON from panthor_submit_ctx_add_job().
- Fix typos in comments.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Steven Price <steven.price@arm.com>