i965/miptree: Allocate HiZ up-front

i965/miptree: Allocate HiZ up-front

HiZ, like MCS and CCS_E, can compress more than just clear colors so we
want it turned on whenever the miptree is being used as a depth
attachment.  It's theoretically possible for someone to create a depth
texture, upload data with glTexSubImage2D, and texture from it without
ever binding it as a depth target.  If this happens, we would end up
wasting a bit of space by allocating a HiZ surface we never use.
However, this is rather unlikely out side of test cases, so we're better
off just allocating it up-front.

Reviewed-by: Chad Versace <chadversary@chromium.org>