intel: Fix intel_unmap_renderbuffer_s8()

intel: Fix intel_unmap_renderbuffer_s8()

When gathering the temporary buffer's pixles into the gem buffer, we had
the two buffers juxtaposed. Oops.

Fixes the following Piglit tests on gen7:
   general/GL_SELECT - alpha-test enabled
   general/GL_SELECT - depth-test enabled
   general/GL_SELECT - no test function
   general/GL_SELECT - scissor-test enabled
   general/GL_SELECT - stencil-test enabled

Fixes SIGABRT in Piglit tests EXT_framebuffer_object/fbo-stencil-* on
gen7.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>