xe: use WC memtype only for SCANOUT

Prefer WB memtype unless passed SCANOUT flag (then use WC).

BUG=b:358427077
TEST=drm-tests

Change-Id: Idc35fb901f176013b921d292c7a946f19b7ac826
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/6245163
Reviewed-by: Matt Turner <msturner@google.com>
Reviewed-by: Dominik Behr <dbehr@chromium.org>
Tested-by: Ryan Neph <ryanneph@google.com>
Commit-Queue: Ryan Neph <ryanneph@google.com>
Reviewed-by: Lina Versace <linyaa@google.com>