MAINTAINERS: Update drm/ci maintainers

Update drm/ci maintainer entries:

* Add myself as drm/ci maintainer.
* Update Helen's email address.

Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>
Acked-by: Helen Koike <helen.fornazier@gmail.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250205134811.2002718-1-vignesh.raman@collabora.com