i965: Use intel_bufferobj_buffer() wrapper in image surface state setup.

Instead of directly using intel_obj->buffer.  Among other things
intel_bufferobj_buffer() will update intel_buffer_object::
gpu_active_start/end, which are used by glBufferSubData() to decide
which path to take.  Fixes a failure in the Piglit
ARB_shader_image_load_store-host-mem-barrier Buffer Update/WaW tests,
which could be reproduced with a non-standard glGetTexSubImage
implementation (see bug report).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105351


Reported-by: Nanley Chery <nanleychery@gmail.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
(cherry picked from commit 936cd3c8)