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