drm/xe/migrate: Add helper function to program identity map

Add an helper function to program identity map.

v2: Formatting nits

Signed-off-by: Akshata Jahagirdar <akshata.jahagirdar@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/91dc05f05bd33076fb9a9f74f8495b48d2abff53.1721250309.git.akshata.jahagirdar@intel.com