drm/connector: Create a helper to attach the hdr_output_metadata property

All the drivers that implement HDR output call pretty much the same
function to initialise the hdr_output_metadata property, and while the
creation of that property is in a helper, every driver uses the same
code to attach it.

Provide a helper for it as well

Signed-off-by: Maxime Ripard <maxime@cerno.tech>