drm/exynos: re-arrange Kconfig entries

Exynos DRM driver have quite big number of components and options.
The patch re-arranges them into three logical groups:
- CRTCs,
- Encoders and Bridges,
- Sub-drivers.
It should make driver options more clear.

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