nvk: Use the same lock for the submit and the memory objects list

We need to lock the memory objects list around the entire push builder process. Otherwise, a memory object could get destroyed between adding it to the BO list and doing the submit, leading to the kernel rejecting our pushbuf.

When the time comes that we switch to the new uAPI both uses of nvk_device::mutex will go away. We'll no longer be passing lists of BOs to the kernel and we'll move to syncobj and be able to drop all the nvk_bo_sync nonsense.