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