drm/vmwgfx: update MAINTAINERS entry

Maintainer switch from Thomas Hellstrom to Roland Scheidegger

Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Neha Bhende <bhenden@vmware.com>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Roland Scheidegger <sroland@vmware.com>