i965: Document conventions for counting layers in 2D multisample buffers.

i965: Document conventions for counting layers in 2D multisample buffers.

The "layer" parameters used in blorp, and the
intel_renderbuffer::mt_layer field, represent a physical layer rather
than a logical layer.  This is important for 2D multisample arrays on
Gen7+ because the UMS and CMS multisample layouts use N physical
layers to represent each logical layer, where N is the number of
samples.

Also add an assertion to blorp to help catch bugs if we fail to follow
these conventions.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>