i965: Fix PBO cache coherency issue after _mesa_meta_pbo_GetTexSubImage().

i965: Fix PBO cache coherency issue after _mesa_meta_pbo_GetTexSubImage().

This problem can easily be reproduced with a number of
ARB_shader_image_load_store piglit tests, which use a buffer object as
PBO for a pixel transfer operation and later on bind the same buffer
to the pipeline as shader image -- The problem is not exclusive to
images though, and is likely to affect other kinds of buffer objects
that can be bound to the 3D pipeline, including vertex, index,
uniform, atomic counter buffers, etc.

CC: 10.5 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>