drm/radeon: insist on 32-bit DMA for Cedar on PPC64/PPC64LE

drm/radeon: insist on 32-bit DMA for Cedar on PPC64/PPC64LE

In radeon_device_init, set the need_dma32 flag for Cedar chips
(e.g. FirePro 2270).  This fixes, or at least works around, a bug
on PowerPC exposed by last year's commits

5940d76a7463b52630b4ad1a6aedc66a08ab97e4 (Russell Currey)

and

0c703bca39114cc8334ea117e512a5b06a2f67f4 (Alistair Popple)

which enabled the 64-bit DMA iommu bypass.

This caused the device to freeze, in some cases unrecoverably, and is
the subject of several bug reports internal to Red Hat.

Signed-off-by: Ben Crocker <bcrocker@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org