i965: Handle non-zero texture buffer offsets in buffer object range calculation.

Otherwise the specified surface state will allow the GPU to access
memory up to BufferOffset bytes past the end of the buffer.  Found by
inspection.

v2: Protect against out-of-range BufferOffset (Nanley).
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
(cherry picked from commit e989acb0)