drm/bridge: analogix_dp: Don't create useless connectors

If there is another bridge after analogix_dp, then the connector object
should not be created. This fixes following timeouts on Exynos5420-based
Chromebook2 Peach-PIT board during boot:

exynos-dp 145b0000.dp-controller: AUX CH cmd reply timeout!
exynos-dp 145b0000.dp-controller: AUX CH enable timeout!
exynos-dp 145b0000.dp-controller: AUX CH enable timeout!
exynos-dp 145b0000.dp-controller: AUX CH enable timeout!
exynos-dp 145b0000.dp-controller: AUX CH enable timeout!

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180305085741.18896-4-m.szyprowski@samsung.com