x86: queensbay: Return directly if IGD / SDVO were already disabled

Initialize 'igd' and 'sdvo' to NULL so that we just need to test
them against NULL later, to be compatible with that case that IGD
and SDVO devices were already in disabled state.

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