drm/xe/uapi: Document use of size in drm_xe_device_query

Document the behavior of the driver for IOCTL DRM_IOCTL_XE_DEVICE_QUERY
depending on the size value provided in struct drm_xe_device_query.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Mateusz Naklicki <mateusz.naklicki@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>