mesa: Add new ctx->Stencil._WriteEnabled derived state flag.

mesa: Add new ctx->Stencil._WriteEnabled derived state flag.

i965 needs to know whether stencil writes are enabled in several places,
and gets the test wrong sometimes.  While we could create a function to
compute this, it seems generally useful enough to warrant a new piece of
derived state.  Also, all the plumbing is already in place.

NOTE: This is a candidate for stable branches.
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>