i965: Only enable depth IZ signals if there's an actual depthbuffer.

i965: Only enable depth IZ signals if there's an actual depthbuffer.

According to the G45 PRM Volume 2 Page 265 we're supposed to only set
these signals when there is an actual depth buffer.  Note that we
already do this for the stencil buffer by virtue of brw->stencil_enabled
invoking _mesa_is_stencil_enabled(ctx) which checks whether the current
drawbuffer's visual has stencil bits (which is updated based on what
buffers are bound).  We just need to do it for depth as well.

Not observed to fix anything.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>