MAINTAINERS: Add drm/xen-front maintainer entry

Add myself as drivers/gpu/drm/xen maintainer.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180403123238.19294-1-andr2000@gmail.com