Added getDefaultDeviceHierarchy call that describes default device
hierarchy for a gfx core. Refactored L0 and OCL paths to use this
value by default and override this value when user sets
ZE_FLAT_DEVICE_HIERARCHY environment variable or
ReturnSubDevicesAsApiDevices debug key.
Updated ReturnSubDevicesAsApiDevices to force COMPOSITE device hierarchy
when set to 0.
Signed-off-by: Filip Hazubski <filip.hazubski@intel.com>