drm/xe/uapi: Remove DRM_XE_VM_BIND_FLAG_ASYNC comment left over

drm/xe/uapi: Remove DRM_XE_VM_BIND_FLAG_ASYNC comment left over

This is a comment left over of commit d3d767396a02
("drm/xe/uapi: Remove sync binds").

Fixes: d3d767396a02 ("drm/xe/uapi: Remove sync binds")
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231226172321.61518-1-jose.souza@intel.com
(cherry picked from commit f031c3a7af8ea06790dd0a71872c4f0175084baa)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>