staging: imx-drm: imx-tve: Fix a sparse warning

This patch declares the function of_get_tve_mode
as a static one to fix this sparse warning:
drivers/staging/imx-drm/imx-tve.c:563:11: warning: \
symbol 'of_get_tve_mode' was not declared. \
Should it be static?

Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>