drm/tilcdc: make tilcdc_mode_hvtotal() static

The function tilcdc_mode_hvtotal is local to the source and does not need
to be in global scope, so make it static.

drivers/gpu/drm/tilcdc/tilcdc_crtc.c:297:6: warning: no previous prototype for 'tilcdc_mode_hvtotal' [-Wmissing-prototypes]
 uint tilcdc_mode_hvtotal(const struct drm_display_mode *mode)

Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>