drm/exynos: exynos7_drm_decon: Consstify struct decon_data

static 'struct decon_data' is only read, so it can be const for code
safety.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>