drm/vmwgfx: fix the check when to use dma_alloc_coherent

Since Linux 4.21 we merged the swiotlb ops into the DMA direct ops,
so they would always have a the sync_single methods.  But late in
the cicle we also removed the direct ops entirely, so we'd see NULL
DMA ops.  Switch vmw_dma_select_mode to only detect swiotlb presence
using swiotlb_nr_tbl() instead.

Fixes: 55897af6 ("dma-direct: merge swiotlb_dma_ops into the dma_direct code")
Fixes: 356da6d0 ("dma-mapping: bypass indirect calls for dma-direct")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>