Your soulution is almost right.

EDITED:

You have to change a detail in the overclock option too.

Details are in the ASRock FAQ:

http://www.asrock.com/support/faq.asp?id=464

(this is for modifying the size, if you want minimal memory usage use 64Mb)

Afther a reboot you see the memory is freed. But there is the Vega11 in devices.

Now you go into BIOS again, and this time use disable instead of force.

And save reboot.

I tested this works.

You have already tried most of the usual recommendations, most notably:

amd cbs\nbio\gfx\integrated graphics controller: forces + uma_specified + 64M (default 1G)  

That typically resolves the issue. Check through the BIOS for any similar settings, if there is another instance to set share memory it may be overriding this one. 

Beyond that, clear CMOS and set the share memory size to 64mb again and see if that helps. If that doesn't work, send a support ticket to ASRock from their website.