use generic dma-direct and swiotlb code for x86 V3

[Christoph Hellwig]

Hi all,

this series switches the x86 code the the dma-direct implementation
for direct (non-iommu) dma and the generic swiotlb ops.  This includes
getting rid of the special ops for the AMD memory encryption case and
the STA2x11 SOC.  The generic implementations are based on the x86
code, so they provide the same functionality.

Changes since V2:
 - minor changelog tweaks
 - made is_sta2x11 a plain bool

Changes since V1:
 - fix the length in the set_memory_decrypted call
 - fix the SEV case