i965: Only do depth resolves prior to clearing when needed

i965: Only do depth resolves prior to clearing when needed

When changing the clear value, we need to resolve any fast cleared data.

Previously, we were performing resolves on every slice with HiZ enabled.
We only need to resolve slices that a) have fast clear data, and b)
aren't about to be cleared to the new color.  In the latter case, we
were actually doing a resolve, and then a fast clear - when we could
skip both, causing the existing fast cleared area to be updated to the
new clear value for no additional work.

This patch stops using intel_miptree_prepare_access in favor of a more
optimal open coded loop that knows about our clear operation.

v2: (by Ken) Rebase on islification, write a real commit message.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>