drm/exynos: Properly report supported formats for each device

Exynos DRM reported that all planes for all supported sub-devices supports
only three pixel formats: XRGB24, ARGB24 and NV12. This patch lets each
Exynos DRM sub-drivers to provide the list of supported pixel formats
and registers this list to DRM core.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>