i965: don't clear resolve map when doing fast depth clears.

i965: don't clear resolve map when doing fast depth clears.

Previously, when performing a fast depth clear, we would also clear
the miptree's resolve map.  This destroyed important information,
since the resolve map contains information about needed resolves for
all levels and layers of the miptree, whereas a depth clear only
applies to a single level/layer combination at a time.  As a result,
resolves would sometimes fail to occur, leading to incorrect
rendering.

Fixes rendering artifacts with shadow maps in Unigine Heaven and
Unigine Sanctuary.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50270

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>