drm/exynos/decon: Add include guard to the Exynos7 header

Although header is included only once but still having an include guard
is a good practice.  To avoid confusion, add SoC prefix to existing
Exynos5433 header include guard.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>