swr: allocate all scratch space in one go for vertex buffers

swr: allocate all scratch space in one go for vertex buffers

Multiple buffers may reference client arrays. When this happens, we
might reach for scratch space multiple times, which could cause later
arrays to invalidate the pointers allocated for the earlier ones.

This fixes copyteximage 2D_ARRAY.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>