drm/exynos: drm_connector: Fix error check condition

drm_add_edid_modes() returns 0 upon failure to find any modes.
Hence check for 0 and not less than 0.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>