drm/vmwgfx: Fix dma API layer violation

Remove the check for IOMMU presence since it was considered a
layer violation.
This means we have no reliable way to destinguish between coherent
hardware IOMMU DMA address translations and incoherent SWIOTLB DMA
address translations, which we can't handle. So always presume the
former. This means that if anybody forces SWIOTLB without also setting
the vmw_force_coherent=1 vmwgfx option, driver operation will fail,
like it will on most other graphics drivers.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>