i965: Move the common binding table offset code to brw_shader.cpp.

i965: Move the common binding table offset code to brw_shader.cpp.

Now that both vec4 and fs are dynamically assigning offsets, a lot of the
code is the same.

v2: Avoid passing around the next offset through the class.  (Review by
    Paul)

Reviewed-by: Paul Berry <stereotype441@gmail.com>