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: NInki Dae <inki.dae@samsung.com>
Reviewed-by: NGustavo Padovan <gustavo.padovan@collabora.co.uk>

drivers/gpu/drm/exynos/Kconfig

@@ -26,7 +26,7 @@ config DRM_EXYNOS_FIMD
 
 config DRM_EXYNOS7_DECON
 	bool "Exynos DRM DECON"
-	depends on DRM_EXYNOS
+	depends on DRM_EXYNOS && !FB_S3C
 	select FB_MODE_HELPERS
 	help
 	  Choose this option if you want to use Exynos DECON for DRM.