drm/exynos: Make mixer_check_timing static

Fixes the following sparse warning:
drivers/gpu/drm/exynos/exynos_mixer.c:821:5: warning:
symbol 'mixer_check_timing' 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>