This is similar to intel_miptree_map_blit and intel_buffer_object.c's
temporary blits in i965.

Improves performance of DiRT Rally by 20-25% by eliminating stalls.

Breaks piglit's spec/arb_shader_image_load_store/host-mem-barrier,
by using the GPU to do uploads, exposing a st/mesa issue where it
doesn't give us memory_barrier() calls.  This is a pre-existing issue
and will be fixed by a later patch (currently out for review).