drm/exynos: remove DRM_EXYNOS_DMABUF config

The exynos drm driver has DRIVER_PRIME capability, then it's reasonable
to support dmabuf as default. Remove DRM_EXYNOS_DMABUF config, it will
prevent that user selects the option unnecessarily.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>