drm/i2c/tda998x: mark symbol static where possible

We get 1 warning when building kernel with W=1:
drivers/gpu/drm/i2c/tda998x_drv.c:1292:5: warning: no previous prototype for 'tda998x_audio_digital_mute' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
So this patch marks this function with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1477127682-3615-1-git-send-email-baoyou.xie@linaro.org