i965/blorp: Use the correct ISL format for combined depth/stencil

i965/blorp: Use the correct ISL format for combined depth/stencil

In brw_blorp_copyteximage, we use the format from the render buffer.
This could be a combined depth/stencil format.  In this case, we handle
stencil properly but we give blorp the wrong ISL format.  Specifically,
we would give blorp ISL_FORMAT_R32G32B32A32_FLOAT which is the wrong
size was causing GPU hangs.

Fixes: GL45-CTS.gtf30.GL3Tests.packed_depth_stencil.packed_depth_stencil_copyteximage

Reviewed-by: Chad Versace <chadversary@chromium.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Cc: "13.0 17.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 4c180f9633421a526f3ea6746cf38b809e7c1abb)