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

This patch resovles the infinite loop issue incurred
when Exyno drm driver is enabled but all kms drivers
are disabled on Exynos board by returning -EPROBE_DEFER
only in case that there is kms device registered.

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