Comment 56 for bug 1428121

Did you try intel_iommu=on,igfx_off boot option? This allows IOMMU on other devices but disables the broken graphics (and HDMI audio).

Regarding the buffer size, try snoop=false option for snd-hda-intel, at first.
The restriction of preallocation size is not real solution, it just happens to work, and inappropriate to put into the driver statically as a quirk.