fix radeon and amdgpu for addressing limited root ports

[Christoph Hellwig]

Hi AMD graphics maintainers,

this series fixes a problem in the radeon driver for systems where the
PCIe root port only supports limited (32-bit) addressing as reported
by Atish.  I then also fixed the same issue in amdgpu as the code was
copy and pasted there, and cleaned up the dma mask setting while
touching that area.