i965: Stop using wm_prog_data->binding_table.render_target_start.

i965: Stop using wm_prog_data->binding_table.render_target_start.

Render target surfaces always start at binding table index 0.
This is required for us to use headerless FB writes, which we
really want to do.  So, we'll never change that.

Given that, it's not necessary to look up a wm_prog_data field
which we already know contains 0.  We can drop the dependency in
brw_renderbuffer_surfaces (Gen4-5)...which was already confusingly
missing from gen6_renderbuffer_surfaces.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>