drm/i915/execlists: Enable coarse preemption boundaries for gen8

When we introduced preemption, we chose to keep it disabled for gen8 as
supporting preemption inside GPGPU user batches required various w/a in
userspace. Since then, the desire to preempt long queues of requests
between batches (e.g. within busywaiting semaphores) has grown. So allow
arbitration within the busywaits and between requests, but disable
arbitration within user batches so that we can preempt between requests
and not risk breaking GPGPU.

However, since this preemption is much coarser and doesn't interfere
with userspace, we decline to include it amongst the scheduler
capabilities. (This is also required for us to skip over the preemption
selftests that expect to be able to preempt user batches.)

Michal suggested that we could perhaps allow preemption inside gen8
userspace batches if we can satisfy ourselves that the default
preemption settings are viable with existing userspace (principally
OpenCL which already should carry any known workaround). We could then
merge the two code paths back into one, even dropping the artifical
has-preemption device feature flag.

Testcase: igt/gem_exec_scheduler/semaphore-user
References: beecec90 ("drm/i915/execlists: Preemption!")
Fixes: e8861964

 ("drm/i915: Use HW semaphores for inter-engine synchronisation on gen8+")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Michal Winiarski <michal.winiarski@intel.com> #irc
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190329134024.5254-1-chris@chris-wilson.co.uk