venus: prefer u_gralloc CROS backend
What does this MR do and why?
venus: prefer u_gralloc CROS backend
Let venus pick cros backend when available.
- On integrations with classic virgl resource, cros backend is able to
query host resource info via private cros gralloc api.
- On integrations with cross-domain resource, those in the guest
gralloc4 buffer metadata are already the real host info. Either
imapper4 or cros api would work.
So we'd try with cros backend first and then fallback to auto pick.
Fixes: 407fd1e1d73 ("venus/android: Switch to using u_gralloc")
Signed-off-by: Yiwei Zhang <zzyiwei@chromium.org>