drm/doc/rfc: Xe is using drm_exec, so mark as completed

Nothing else to be done on this front from Xe perspective.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231201042158.80009-6-rodrigo.vivi@intel.com
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>