vapostproc: Make cb max values symmetrical to their min values.

Intel drivers expose some colorbalance's maximum values much more bigger than their minimum values, given their middle values (default value). This means, in practice, that the real middle point between the maximum and minimum values implies a major change in the color balance, which is not expected by the GStreamer color balance logic.

This patch makes the given maximum value symmetrical to the minimum value, given the middle one (default value).