drm/i915: Skip an engine reset if it recovered before our preparations

At the beginning of a reset, we disable the submission method and find
the stuck request. We expect to find a stuck request for we have
declared the engine stalled. However, if we find no active request, the
engine must have recovered from its stall before we could issue a reset,
so let the engine continue on without a reset. If the engine is truly
stuck, we will back soon enough with the next reset attempt.

v2: Remove the stale debug message.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171216002206.31737-1-chris@chris-wilson.co.uk