drm/exynos: Remove support for Exynos4415 (SoC not supported anymore)

Support for Exynos4415 is going away because there are no internal nor
external users.

Since commit 46dcf0ff ("ARM: dts: exynos: Remove exynos4415.dtsi"),
the platform cannot be instantiated so remove also the drivers.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Kukjin Kim <kgene@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>