drm/exynos: add a dependency on FB_S3C to DECON driver

This patch makes one of Linux framebuffer and DRM CRTC drivers
to be enabled.

Display controllers, FIMD and DECON, can be controlled by Linux
framebuffer or DRM CRTC drivers so only one of them should be
enabled.

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