drm/mediatek: mark symbols static where possible

We get 4 warnings when building kernel with W=1:
drivers/gpu/drm/mediatek/mtk_hdmi.c:1089:6: warning: no previous prototype for 'mtk_hdmi_audio_enable' [-Wmissing-prototypes]
drivers/gpu/drm/mediatek/mtk_hdmi.c:1095:6: warning: no previous prototype for 'mtk_hdmi_audio_disable' [-Wmissing-prototypes]
drivers/gpu/drm/mediatek/mtk_hdmi.c:1101:5: warning: no previous prototype for 'mtk_hdmi_audio_set_param' [-Wmissing-prototypes]
drivers/gpu/drm/mediatek/mtk_hdmi.c:1627:5: warning: no previous prototype for 'mtk_hdmi_audio_digital_mute' [-Wmissing-prototypes]

In fact, both functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
So this patch marks both functions with 'static'.
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
[seanpaul fixed checkpatch warning for argument alignment]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1474789109-22010-2-git-send-email-baoyou.xie@linaro.org

drivers/gpu/drm/mediatek/mtk_hdmi.c

@@ -1086,19 +1086,19 @@ static int mtk_hdmi_output_init(struct mtk_hdmi *hdmi)
 	return 0;
 }
 
-void mtk_hdmi_audio_enable(struct mtk_hdmi *hdmi)
+static void mtk_hdmi_audio_enable(struct mtk_hdmi *hdmi)
 {
 	mtk_hdmi_aud_enable_packet(hdmi, true);
 	hdmi->audio_enable = true;
 }
 
-void mtk_hdmi_audio_disable(struct mtk_hdmi *hdmi)
+static void mtk_hdmi_audio_disable(struct mtk_hdmi *hdmi)
 {
 	mtk_hdmi_aud_enable_packet(hdmi, false);
 	hdmi->audio_enable = false;
 }
 
-int mtk_hdmi_audio_set_param(struct mtk_hdmi *hdmi,
+static int mtk_hdmi_audio_set_param(struct mtk_hdmi *hdmi,
 				    struct hdmi_audio_param *param)
 {
 	if (!hdmi->audio_enable) {
@@ -1624,7 +1624,8 @@ static void mtk_hdmi_audio_shutdown(struct device *dev, void *data)
 	mtk_hdmi_audio_disable(hdmi);
 }
 
-int mtk_hdmi_audio_digital_mute(struct device *dev, void *data, bool enable)
+static int
+mtk_hdmi_audio_digital_mute(struct device *dev, void *data, bool enable)
 {
 	struct mtk_hdmi *hdmi = dev_get_drvdata(dev);