Fix the following sparse warning:

drivers/gpu/drm/i2c/tda998x_drv.c:1136:5: warning:
symbol 'tda998x_audio_digital_mute' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Samuel Zou <zou_wei@huawei.com>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1588819768-11818-1-git-send-email-zou_wei@huawei.com