drm/xe/client: drop bogus bo NULL check

If we fished it out the list then it can't be null; the list entry is
embedded in the bo.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240318093431.21075-4-matthew.auld@intel.com