iris: Lock the BO deps lock around all of batch submission

Otherwise, if we have a mean state tracker (say, OpenCL) thrashing multiple contexts from multiple threads on the same screen, there's a race between when the syncobjs for a BO are adjusted and when we actually submit to the kernel. If we lose this race in the right way, we can end up with one batch trying to wait on a syncobj before the corresponding signalling batch has been submitted to the kernel, leading to a -EINVAL from i915.

Fixes: 89a34cb8 ("iris: switch to explicit busy tracking")