drm/i915/gt: Extract busy-stats for ring-scheduler

Lift the busy-stats context-in/out implementation out of intel_lrc, so
that we can reuse it for other scheduler implementations.

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/20210115142331.24458-2-chris@chris-wilson.co.uk