drm/xe: Fix error handling if PXP fails to start

drm/xe: Fix error handling if PXP fails to start

Since the PXP start comes after __xe_exec_queue_init() has completed,
we need to cleanup what was done in that function in case of a PXP
start error.
__xe_exec_queue_init calls the submission backend init() function,
so we need to introduce an opposite for that. Unfortunately, while
we already have a fini() function pointer, it performs other
operations in addition to cleaning up what was done by the init().
Therefore, for clarity, the existing fini() has been renamed to
destroy(), while a new fini() has been added to only clean up what was
done by the init(), with the latter being called by the former (via
xe_exec_queue_fini).

Fixes: 72d479601d67 ("drm/xe/pxp/uapi: Add userspace and LRC support for PXP-using queues")
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://lore.kernel.org/r/20250909221240.3711023-3-daniele.ceraolospurio@intel.com
(cherry picked from commit 626667321deb4c7a294725406faa3dd71c3d445d)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>