drm/xe: Allow error injection for xe_pxp_exec_queue_add

This will allow us to simulate this function returning an error like
we do for other functions called in the exec_queue_create path.

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>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://lore.kernel.org/r/20250909221240.3711023-4-daniele.ceraolospurio@intel.com