drm/exynos: fix the input prompt of Exynos7 DECON

This patch is a preparation patch for adding support for Exynos5433
DECON. Exynos7 DECON have to be distinguished from Exynos5433 DECON.

Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>