drm/vc4: vec: Add the margin properties to the connector

All the handling for the properties was present, but they
were never attached to the connector to allow userspace
to change them.

Add them to the connector.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240619153913.2804051-4-dave.stevenson@raspberrypi.com