drm/i915: Only recover active engines

If we issue a reset to a currently idle engine, leave it idle
afterwards. This is useful to excise a linkage between reset and the
shrinker. When waking the engine, we need to pin the default context
image which we use for overwriting a guilty context -- if the engine is
idle we do not need this pinned image! However, this pinning means that
waking the engine acquires the FS_RECLAIM, and so may trigger the
shrinker. The shrinker itself may need to wait upon the GPU to unbind
and object and so may require services of reset; ergo we should avoid
the engine wake up path.

The danger in skipping the recovery for idle engines is that we leave the
engine with no context defined, which may interfere with the operation of
the power context on some older platforms. In practice, we should only
be resetting an active GPU but it something to look out for on Ironlake
(if memory serves).

Fixes: 79ffac85

 ("drm/i915: Invert the GEM wakeref hierarchy")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626154549.10066-2-chris@chris-wilson.co.uk