drm/exynos: resolve infinite loop issue on multi-platform

This patch resolves temporarily infinite loop issue incurred
when Exynos drm driver is enabled and multi-platform kernel
is used by registering Exynos drm device object only in case
of Exynos SoC. So this patch will be replaced with more generic
way later.

Signed-off-by: Inki Dae <inki.dae@samsung.com>