i965/Gen7: Set up layer constraints properly for renderbuffers

i965/Gen7: Set up layer constraints properly for renderbuffers

There were a few problems here, which mostly just broke layered
rendering into a view:

- Render target view extent was always set to be == depth. This is
  benign for non-layered-rendering, but allows writes off the end of the
  render target for layered rendering, which ends badly.
- Layered rendering did not honor the mt_layer setting, so would not
  properly handle MinLayer being set on a view.

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>