drm/i915: Bump GT idling delay to 2 jiffies

In monitoring a transcode pipeline that is latency sensitive (it waits
between submitting frames, and each frame requires work on rcs/vcs/vecs
engines), it is found that it took longer than a single jiffy for it to
sustain its workload. Allowing an extra jiffy headroom for the userspace
prevents us from prematurely parking and having to exit powersaving
immediately.

Link: https://gitlab.freedesktop.org/drm/intel/-/issues/6284
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Karolina Drobnik <karolina.drobnik@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e37911ec087a9ce50630d6faf61fa2c0d5f96d44.1657289332.git.karolina.drobnik@intel.com