drm/xe/guc_submit: fixup deregister in job timeout

Rather check if the engine is still registered before proceeding with
deregister steps. Also the engine being marked as disabled doesn't mean
the engine has been disabled or deregistered from GuC pov, and here we
are signalling fences so we need to be sure GuC is not still using this
context.

v2:
 - Drop the read_stopped() for this path. Since we are signalling
   fences on error here, best play it safe and wait for the GT reset to
   mark the engine as disabled, rather than it just being queued.
v3 (Matt Brost):
 - Keep the read_stopped() on the wait event, since there is no need to
   wait for an already scheduled GT reset. If it is set we can then just
   bail without signalling anything.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>