Based on present mode, different values for the image count make sense.
In mailbox mode, the driver will increase the number as necessary.  In
immediate mode, two buffers suffice.  In fifo mode, we would ideally
want triple-buffering.

Bug: angleproject:2932
Change-Id: I45eec83d0292eea472b931fe81e49cde6d105c27
Reviewed-on: https://chromium-review.googlesource.com/c/1454290Reviewed-by: Ian Elliott <ianelliott@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>