i965/miptree: Set supports_fast_clear = false in make_shareable

The make_shareable function deletes the aux buffer and then whacks
aux_usage to ISL_AUX_USAGE_NONE but not unsetting supports_fast_clear.
Since we only look at supports_fast_clear to decide whether or not to do
fast clears, this was causing assertion failures.

Reported-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101925
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
(cherry picked from commit e7a52cc381)

src/mesa/drivers/dri/i965/intel_mipmap_tree.c

@@ -2720,6 +2720,7 @@ intel_miptree_make_shareable(struct brw_context *brw,
    }
 
    mt->aux_usage = ISL_AUX_USAGE_NONE;
+   mt->supports_fast_clear = false;
 }