drm/amdgpu: simplify and cleanup setting the dma mask

Use dma_set_mask_and_coherent to set both masks in one go, and remove
the no longer required fallback, as the kernel now always accepts
larger than required DMA masks.  Fail the driver probe if we can't
set the DMA mask, as that means the system can only support a larger
mask.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>