drm/xe/bo: explicitly reject zero sized BO

In the depths of ttm, when allocating the vma node this should result in
-ENOSPC it seems. However we should probably rather reject as part of
our own ioctl sanity checking, and then treat as programmer error in the
lower levels.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>