drm: exynos: dsi: Restore proper bridge chain order

Restore the proper bridge chain by finding the previous bridge
in the chain instead of passing NULL.

This establishes a proper bridge chain while attaching downstream
bridges.

Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>