viafb: select HW scaling on VX900 for IGA2

VX900 can do hardware scaling for both IGAs in contrast to previous
hardware which could do it only for IGA2. This patch ensures that
we set the parameter for IGA2 and not for IGA1. This fixes hardware
scaling on VX900 until we have the infrastructure to support it for
both IGAs.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: stable@vger.kernel.org