DSI device can be attached after DRM device is registered. In such
case newly created connector must be registered by exynos_dsi.
The patch fixes exynos_drm on rinato and trats boards.

Fixes: 6afb7721 ("drm/exynos: move connector creation to attach callback")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>