drm/meson: Make some functions static

Fix sparse warnings:

drivers/gpu/drm/meson/meson_viu.c:93:6: warning: symbol 'meson_viu_set_g12a_osd1_matrix' was not declared. Should it be static?
drivers/gpu/drm/meson/meson_viu.c:121:6: warning: symbol 'meson_viu_set_osd_matrix' was not declared. Should it be static?
drivers/gpu/drm/meson/meson_viu.c:190:6: warning: symbol 'meson_viu_set_osd_lut' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190413141455.34020-1-yuehaibing@huawei.com