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>

src/intel/blorp/blorp.c

@@ -176,8 +176,10 @@ blorp_compile_fs(struct blorp_context *blorp, void *mem_ctx,
    wm_prog_data->base.nr_params = 0;
    wm_prog_data->base.param = NULL;
 
-   /* BLORP always just uses the first two binding table entries */
-   wm_prog_data->binding_table.render_target_start = BLORP_RENDERBUFFER_BT_INDEX;
+   /* BLORP always uses the first two binding table entries:
+    * - Surface 0 is the render target (which always start from 0)
+    * - Surface 1 is the source texture
+    */
    wm_prog_data->base.binding_table.texture_start = BLORP_TEXTURE_BT_INDEX;
 
    nir = brw_preprocess_nir(compiler, nir);