[Intel-gfx] Execlists preemption & user priority

[Chris Wilson]

Some refinement from the earlier version, as Michal pointed some
workarounds we needed to prevent an obstructive break in ABI.
Preemption granularity affects how userspace should consider building
batches, so we need to match sure that as we switch preemption on, the
granulatity doesn't change. Sadly, this means we can't enable preemption
on BDW/BSW unconditionally. At least not at this time.

The usecase for this naked series is to enable preemption for pageflips.
It is also exposed to userspace via a context priority, but we reserve
for CAP_SYS_NICE processes for now as we have a very, very raw and
unfair scheduler. But it should enable display servers to squeeze that
last bit of vblank goodness out of the system.
-Chris