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

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 4e5306453da6a1c076309e543ec92d999e02f67a
("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