drm/tilcdc: mark symbols static where possible

We get 3 warnings when building kernel with W=1:
drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c:142:29: warning: no previous prototype for 'tilcdc_get_overlay' [-Wmissing-prototypes]
drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c:198:13: warning: no previous prototype for 'tilcdc_convert_slave_node' [-Wmissing-prototypes]
drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c:264:12: warning: no previous prototype for 'tilcdc_slave_compat_init' [-Wmissing-prototypes]

In fact, these 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 these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Jyri Sarha <jsarha@ti.com>