vgaarb: Keep adding VGA device in queue

If failed to find the deivice owning the boot framebuffer,
try to use the first VGA device instead of the last one.

Usually the 1st device is integrated GPU who owns the boot framebuffer.

Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1535739600-8842-2-git-send-email-aaron.ma@canonical.com