i965: Properly demote the depth mt format for fake packed depth/stencil.

i965: Properly demote the depth mt format for fake packed depth/stencil.

gen7 only supports the non-packed formats, even if you associate a
real separate stencil buffer -- otherwise it's as if the depth test
always fails.

This requires a little bit of care in the match_texture_image case,
since the miptree format no longer matches the texture image format.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>