[Intel-gfx] Finish gen8 legacy semaphores

[Chris Wilson]

Semaphores for gen8 were almost finished and only required a couple of
tweaks to pass the most stressful igt I could write, as well as the
existing inter-engine stress tests.

But what's the point you might ask when legacy ringbuffer submission is
already turned off by default? gen8 still serves as our test platform on
which we can directly compare the alternative submission modes and for
the purposes of testing, we want it to be as feature complete as
possible.
-Chris