xwayland/present: Move xwl_present_reset_timer call out of xwl_present_flip

xwl_present_reset_timer checks if the pending flip is synchronous, so we need to call it after adding the pending flip to the flip queue.

Fixes: b2a06e07 "xwayland/present: Drop sync_flip member of struct xwl_present_window" Closes: #1219 (closed)