The recent regression report revealed that the use of WC pages for AMD
HDMI device together with AMD IOMMU leads to unexpected truncation or
noises.  The issue seems triggered by the change in the kernel core
memory allocation that enables IOMMU driver to use always S/G
buffers.  Meanwhile, the use of WC pages has been a workaround for the
similar issue with standard pages in the past.  So, now we need to
apply the workaround conditionally, namely, only when IOMMU isn't in
place.

This patch modifies the workaround code to check the DMA ops at first
and apply the snoop-off only when needed.

Fixes: f5ff79fd ("dma-mapping: remove CONFIG_DMA_REMAP")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=219087
Link: https://patch.msgid.link/20240731170521.31714-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>