drm/xe/kunit: Define helper functions to allocate fake xe device

There will be more KUnit tests added that will require fake device.
Define generic helper functions to avoid code duplications.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20231218190629.502-6-michal.wajdeczko@intel.com
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>