drm/i915/gt: Always reset the engine, even if inactive, on execlists failure

If something has gone awry with the CSB processing, we need to pause,
unwind and restart the request submission and event processing. However,
currently we skip the engine reset if we raise an error but discover no
active context, in the mistaken belief that it was merely a glitch in
the matrix. The glitches are real enough, and we do need to unwind even
if the engine appears idle (as it has gone permanently idle!) The
simplest way to unwind and recover is simply do the engine reset, which
should be very fast and _safe_ as nothing is active.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200711091349.28865-1-chris@chris-wilson.co.uk