compositor-drm: ignore case of {h,v}sync flags in modeline

Some modeline generators put out e.g. +HSync instead of +hsync. Accept
that too since it's not ambigous.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>