If an engine is only destroyed on driver unload, we can skip its
clean-up steps with the GuC because the GuC is going to be tuned off as
well, so it doesn't matter if we're in sync with it or not. Currently,
we apply this optimization to all engines marked as kernel, but this
stops us to supporting kernel engines that don't stick around until
unload. To remove this limitation, add a separate flag to indicate if
the engine is expected to only be destryed on driver unload and use that
to trigger the optimzation.

While at it, add a small comment to explain what each engine flag
represents.

v2: s/XE_BUG_ON/XE_WARN_ON, s/ENGINE/EXEC_QUEUE
v3: rebased
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20230822173334.1664332-3-daniele.ceraolospurio@intel.comSigned-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>