drm/connector: Add a helper to attach the colorspace property

The intel driver uses the same logic to attach the Colorspace property
in multiple places and we'll need it in vc4 too. Let's move that common
code in a helper.

Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430094451.2145002-4-maxime@cerno.tech