i965: Drop render_target_start from binding table struct.

i965: Drop render_target_start from binding table struct.

We have to start render targets at binding table index 0 in order to use
headerless FB write messages, and in fact already assume this in a bunch
of places in the code.  Let's finish that off, and not bother storing 0
in a struct to pretend to add it in a few places.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>