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.

See https://bugs.freedesktop.org/show_bug.cgi?id=110422



Signed-off-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 4583f09c)