radeonsi: init sctx->dma_copy before using it
Commit a1378639 reordered context functions initializations but broke sctx->b.resource_copy_region init when using AMD_DEBUG=forcedma.
In this case sctx->dma_copy was assigned a value after being used in: sctx->b.resource_copy_region = sctx->dma_copy;
This commit moves the FORCE_DMA special case after sctx->dma_copy initialization.