drm/exynos: Add missing static storage class specifier

Fixes the following sparse warning:
drivers/gpu/drm/exynos/exynos_drm_connector.c:199:20:
warning: symbol 'exynos_drm_best_encoder' was not declared. Should it be static?
Signed-off-by: NSachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: NInki Dae <inki.dae@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_connector.c

@@ -196,7 +196,8 @@ static int exynos_drm_connector_mode_valid(struct drm_connector *connector,
 	return ret;
 }
 
-struct drm_encoder *exynos_drm_best_encoder(struct drm_connector *connector)
+static struct drm_encoder *exynos_drm_best_encoder(
+						struct drm_connector *connector)
 {
 	struct drm_device *dev = connector->dev;
 	struct exynos_drm_connector *exynos_connector =