iris: Use coherent allocation for PIPE_RESOURCE_STAGING

iris: Use coherent allocation for PIPE_RESOURCE_STAGING

On !llc machines (Atoms), reading from a linear buffers is slow and so
copying from one resource into the linear staging buffer is still slow.
However, we can tell the GPU to snoop the CPU cache when reading from and
writing to the staging buffer eliminating the slow uncached reads.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>