drm/connector: Allow creation of margin props alone

TV margins properties can only be added as part of the SDTV TV
connector properties creation, but we might need those props for HDMI
TVs too, so let's move the margins props creation in a separate
function and expose it to drivers.

We also add an helper to attach margins props to a connector.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181206142439.10441-4-boris.brezillon@bootlin.com