drm/xe/bo: refactor try_add_vram

Get rid of some of the duplication here. In a future patch we need to
also consider [fpfn, lpfn], so better adjust in only one place.

Suggested-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>