gallium/u_transfer_helper: Initialize the stride of MSAA maps.

We just never set the value that was returned for MSAA mappings (directly
reading back an MSAA framebuffer).  Since we're handing back ss_map, it
should be ss_map's stride from our nested transfer.

Fixes piglit /home/anholt/src/piglit/bin/fbo-depthstencil -samples=4
cases.

Reviewed-by: Rob Clark <robdclark@gmail.com>

src/gallium/auxiliary/util/u_transfer_helper.c

@@ -218,6 +218,7 @@ transfer_map_msaa(struct pipe_context *pctx,
       return NULL;
    }
 
+   ptrans->stride = trans->trans->stride;
    *pptrans = ptrans;
    return ss_map;
 }