x86: ivybridge: Use syscon for the GMA device

x86: ivybridge: Use syscon for the GMA device

Until we have a proper video uclass we can use syscon to handle the GMA
device, and avoid the special device tree and PCI searching. Update the code
to work this way.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>