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

drivers/gpu/drm/exynos/exynos_mixer.c

@@ -818,7 +818,7 @@ static void mixer_win_disable(void *ctx, int win)
 	mixer_ctx->win_data[win].enabled = false;
 }
 
-int mixer_check_timing(void *ctx, struct fb_videomode *timing)
+static int mixer_check_timing(void *ctx, struct fb_videomode *timing)
 {
 	struct mixer_context *mixer_ctx = ctx;
 	u32 w, h;