drm/xe/kunit: Return number of iterated devices

In xe_call_for_each_device() we are already counting number of
iterated devices. Lets make that available to the caller too.
We will use that functionality in upcoming patches.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20231115115816.1993-1-michal.wajdeczko@intel.com


Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>