drm/i915/gt: Only disable preemption on gen8 render engines

The reason why we did not enable preemption on Broadwater was due to
missing GPGPU workarounds. Since this only applies to rcs0, only
restrict rcs0 (and our global capabilities).

While this does not affect exposing a preemption capability to
userspace, it does affect our internal decisions on whether to use
timeslicing and semaphores between individual engines.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210108204026.20682-6-chris@chris-wilson.co.uk