intel/isl: Add a helper for getting the byte/tile offset of a subimage

intel/isl: Add a helper for getting the byte/tile offset of a subimage

Frequently, get_image_offset_sa is combined with get_intratile_offset_sa
so it makes sense to have a single helper to do both.  If the caller
doesn't want the intratile offsets, it can simply pass NULL and ISL will
assert that they are 0.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>