drm/xe/pxp: Decouple queue addition from PXP start

Starting PXP and adding a queue to the PXP queue list are separate
actions. Given that a queue can only be added to the list if PXP is
active, the 2 actions were bundled together to avoid having to
re-lock and re-check the status to perform the queue addition after
having done so during the PXP start. However, we don't save a lot of
complexity by doing so and we lose in clarity of code, so overall it's
cleaner to just keep the 2 actions separate.

v2: remove leftover rpm_get (John), fix rpm_put in error case

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://lore.kernel.org/r/20250522225401.3953243-8-daniele.ceraolospurio@intel.com