i965: Correct check for re-bound buffer in intel_update_image_buffer

i965: Correct check for re-bound buffer in intel_update_image_buffer

The buffer-object is the persistent thing passed through the loader, so when
updating an image buffer, check to see if it is already bound to the provided
bo. The region, on the other hand, is allocated separately for the miptree,
and so will never be the same as that passed back from the loader.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>