drm/radeon/kms: enable the ttm dma pool if swiotlb is on V4

With the exception that we do not handle the AGP case. We only
deal with PCIe cards such as ATI ES1000 or HD3200 that have been
detected to only do DMA up to 32-bits.

V2 force dma32 if we fail to set bigger dma mask
V3 Rebase on top of no memory account changes (where/when is my
   delorean when i need it ?)
V4 add debugfs entry is swiotlb is active not only if we are
   on dma 32bits only gpu

CC: Dave Airlie <airlied@redhat.com>
CC: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>