drm/ast: Fixed reboot test may cause system hanged

There is another thread still access standard VGA I/O while loading drm driver.
Disable standard VGA I/O decode to avoid this issue.

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Reviewed-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523410059-18415-1-git-send-email-yc_chen@aspeedtech.com