wsi/x11: Create a present queue for VK_PRESENT_MODE_FIFO_RELAXED_KHR
This activates the target_msc computation that we'd otherwise be using if we were just FIFO; otherwise we'd always set target_msc = 0 and it'd act like IMMEDIATE.
Fixes: 77d25627 "wsi/x11: Hook up VK_PRESENT_MODE_FIFO_RELAXED_KHR" Fixes: #3512 (closed)