i965: Fix stencil write enable flag in 3DSTATE_DEPTH_BUFFER on Gen7+.

i965: Fix stencil write enable flag in 3DSTATE_DEPTH_BUFFER on Gen7+.

ctx->Stencil.WriteMask is a statically sized array of 3 elements.
Checking it against 0 actually is a NULL check, and can never fail,
which meant that we always said stencil writes were enabled.

Use the new core Mesa derived state flag to fix this.

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