drm/xe/uapi: Ensure every uapi struct has drm_xe prefix

To ensure consistency and avoid possible later conflicts,
let's add drm_xe prefix to xe_user_extension struct.

Cc: Francois Dugast <francois.dugast@intel.com>
Suggested-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Mateusz Naklicki <mateusz.naklicki@intel.com>