drm: xlnx: zynqmp_dpsub: Add connected live layer helper

Add a helper function capturing the first connected live display layer
discovery logic.

Signed-off-by: Anatoliy Klymenko <anatoliy.klymenko@amd.com>
Reviewed-by: Tomi Valkeinen <tomi.valkainen@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240416-dp-live-fmt-v4-3-c7f379b7168e@amd.com