drm/sti/sti_hdmi: Move 'colorspace_mode_names' array to where its used

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/sti/sti_hdmi.h:36:40: warning: ‘colorspace_mode_names’ defined but not used [-Wunused-const-variable=]
 36 | static const struct drm_prop_enum_list colorspace_mode_names[] = {
 | ^~~~~~~~~~~~~~~~~~~~~

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201112190039.2785914-28-lee.jones@linaro.org