libvulkan: Un-revert and fix VK_PRESENT_MODE_MAILBOX_KHR

This reverts commit 9ec497c4, which was a revert of f3339223, and fixes
the problem that caused the initial revert.

MODE_MAILBOX corresponds to BufferQueue "async" mode; in that mode we
need one extra un-dequeued buffer at all times. We weren't accounting
for that, and weren't allocating enough bufers.

Bug: 26918467
Change-Id: Ice67e6d16828e50413ae453ecf3c67b8fb288f1a