Revert "drm/exynos: fix null pointer dereference issue"

This reverts commit cea24824ab432f8acabb254d6805e9aa756de6af.

Moving subdriver probe to exynos_drm_platform_probe() was making
exynos_drm_device_subdrv_probe() fail because the platform data wasn't set
yet. It only gets set in exynos_drm_load.

We need to find a smarter way to fix this issue.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>