i965/gen7: Fold WM surface state prepare()/emit() together.

i965/gen7: Fold WM surface state prepare()/emit() together.

These produce BRW_NEW_SURFACES (used by binding table emit()) and
BRW_NEW_NR_WM_SURFACES (used by WM unit emit()).  Fixes a bug where
with no texturing and no color buffer, we wouldn't consider the null
renderbuffer in nr_surfaces.  This was harmless because nr_surfaces is
only used for the prefetch info in the unit state.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Paul Berry <stereotype441@gmail.com>