PCI/P2PDMA: Fix build without DMA ops

My commit to make DMA ops support optional missed the reference in
the p2pdma code.  And while the build bot didn't manage to find a config
where this can happen, Matthew did.  Fix this by replacing two IS_ENABLED
checks with ifdefs.

Fixes: 2f9237d4

 ("dma-mapping: make support for dma ops optional")
Link: https://lore.kernel.org/r/20200810124843.1532738-1-hch@lst.de
Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>