i965/Gen7: Set up layer constraints properly for depth buffers

i965/Gen7: Set up layer constraints properly for depth buffers

Again, a few problems:
- Layered attachments did not honor MinLayer.
- Non-layered MSAA attachments rendered to the wrong layer due to
  dividing by the layer count. All depth buffers use the IMS layout, so
  the physical layer count == logical layer count.
- Layered attachments were not limited to irb->layer_count, so we could
  render off the end of the texture.

V2: Restore the MAX2() to account for zero depth/layer_count.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>