drm/radeon: fixup bad vram size on SI
drm/radeon: fixup bad vram size on SI

From Alex Deucher
7e3f2ed899128187ecc9ab7573e966d8a5afc664 in ubuntu 3.8
0ca223b029a261e82fb2f50c52eb85d510f4260e in mainline linux

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars