drm/exynos: consolidate driver/device initialization code

Code registering different drivers and simple platform devices was dispersed
across multiple sub-modules. This patch moves it to one place. As a result
initialization code is shorter and cleaner and should simplify further
development.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>