drm/exynos: Clean up logs for DRM_ERROR / DRM_DEBUG_KMS

This patch cleans up logs for DRM_ERROR / DRM_DEBUG_KMS to avoid
logging duplicated function name because the macros already contain
 __func__.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>