drm/xe: Add function to check if BO has single placement

A new helper function xe_bo_has_single_placement() to check
if a BO has single placement.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240430162529.21588-5-nirmoy.das@intel.com
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>