drm/tve200: make two functions static

The functions tve200_display_disable and tve200_display_funcs are
local to the source and do not need to be in global scope, so make
them static.

Cleans up sparse warnings:
symbol 'tve200_display_disable' was not declared. Should it be static?
symbol 'tve200_display_funcs' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170922160516.16283-1-colin.king@canonical.com