vulkan: Set shared & auto refresh modes earlier

We'd like to be able to remove the 2-buffer-minimum workaround -- but to
make that reasonable, we need the lower layers to know we're trying to
use shared mode. Set the appropriate modes before determining the buffer
counts, as a first step toward that.

Change-Id: I83a676fc1a86510abd4521cf9a5e80c0334d5941
Test: build