Sorry to resurrect such an old thread, but maybe my comments will be helpful to someone, even if it's only me next time I set up a new FreeBSD system and forgot what I did with this one which seems to have fixed it.
I could be wrong, but it seems to me that this problem was never actually solved and maybe everyone just moved on to USB mice and KVM switches. Well I still have PS/2 KVM switches, 1 at home and 5 at work. The 2 on my workbench at work exhibit this problem and I've been struggling with it for years through numerous versions of FreeBSD. I've tried all the suggestions that I found online and the best I managed with them was intermittent mouse functionality. Rather than waste a lot of time, I usually just ended up sticking a USB mouse on it. Not ideal, because I invariably grabbed the wrong mouse 1st and one or the other of the mice was always in the way.
This time, I guess I was just fed up with that approach so I was a little more persistent. I wasn't planning to mod the driver code, though I did take a look at it. I went through the psm man page to see if there were any config options that might help. There was nothing specific to this problem but there was flag bit 12, IGNOREPORTERROR, with a rather vague description which states 'it should never be necessary under normal circumstances'. I suppose the fact that I'm still using a PS/2 KVM switch in 2020 with FreeBSD 12.1 might be considered an
abnormal circumstance, so I added 'hint.psm.0.flags="0x0800" (the IGNOREPORTERROR bit) to /boot/device.hints. After that I tried a warm boot, a cold boot, and a cold boot with the KVM switch turned to a different port. The mouse was enabled and working after each.
Hope this is of help to someone else and apologies if this solution is posted somewhere else and I failed to find it.