i965: Always reserve binding table space for at least one render target.

In brw_update_renderbuffer_surfaces(), if there are no color draw
buffers, we always set up a null render target at surface index 0 so we
have something to use with the FB write marking the end of thread.

However, when we recently began computing surface indexes dynamically,
we failed to reserve space for it.  This meant that the first texture
would be assigned surface index 0, and our closing FB write would
clobber the texture.

Fixes Piglit's EXT_packed_depth_stencil/fbo-blit-d24s8 test on Gen4-5,
which regressed as of commit 4e5306453d
("i965/fs: Dynamically set up the WM binding table offsets.")

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70605
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Tested-by: lu hua <huax.lu@intel.com>
Cc: "10.0" mesa-stable@lists.freedesktop.org

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -3181,8 +3181,11 @@ fs_visitor::assign_binding_table_offsets()
 {
    uint32_t next_binding_table_offset = 0;
 
+   /* If there are no color regions, we still perform an FB write to a null
+    * renderbuffer, which we place at surface index 0.
+    */
    c->prog_data.binding_table.render_target_start = next_binding_table_offset;
-   next_binding_table_offset += c->key.nr_color_regions;
+   next_binding_table_offset += MAX2(c->key.nr_color_regions, 1);
 
    assign_common_binding_table_offsets(next_binding_table_offset);
 }