PCI/P2PDMA: Allow P2PDMA on AMD Zen and newer CPUs

Allow P2PDMA if the CPU vendor is AMD and family is 0x17 (Zen) or greater.

[bhelgaas: commit log, simplify #if/#else/#endif]
Link: https://lore.kernel.org/r/20200729231844.4653-1-logang@deltatee.com


Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>