port-hp300/54255: Support SGC graphics in any slot in hp300 system

[cmhanson%eschatologist.net@localhost]

>Number:         54255
>Category:       port-hp300
>Synopsis:       Support SGC graphics in any slot in hp300 system
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-hp300-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri May 31 20:45:00 +0000 2019
>Originator:     Chris Hanson
>Release:        netbsd-8
>Organization:
>Environment:
>Description:
Currently the hp300 port's SGC graphics only supports the 9000/425e because it assumes an SGC frame buffer is only in slot 0. On an 9000/425t however, an SGC frame buffer will be in slot 3.

In discussion of this on the port-hp300 list at <http://mail-index.netbsd.org/port-hp300/2018/11/18/msg000126.html>, Izumi Tsutsui suggested that we scan all SGC slots. I've tested the changes to do so on my HP 9000/425t running netbsd-8 and they work fine so I suggest they be incorporated ASAP.
>How-To-Repeat:
Try to boot netbsd-8 on an HP 9000/425t with SGC graphics (HP A1659A card). You will have to use serial console because the graphical console will stop working partway through the boot.
>Fix:
Incorporate Izumi Tsutsui's fixes posted to the port-hp300 list, rebuild the kernel, and boot the new kernel. NetBSD will come right up.