drm/exynos: mixer: remove support for non-dt platforms

There are no non-devicetree based Exynos platforms in mainline, so there
no point keeping old platform driver data for them.

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