i965: Fix incorrect bounds tracking for blit readpixels's GPU access.

i965: Fix incorrect bounds tracking for blit readpixels's GPU access.

While incorrect, it probably wouldn't affect anyone ever: You'd have to do
an appropriately-formatted readpixels into a PBO, then overwrite the tail
end of the updated area of the PBO with glBufferSubData(), and you
wouldn't get appropriate synchronization.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>