xwayland: Use xwl_present_reset_timer in xwl_present_timer_callback

Apart from simplifying the code, this should also prevent a condition
(which might only be possible with the following fix) reported in
https://gitlab.freedesktop.org/wayland/weston/issues/115#note_52467:

1. xwl_present_timer_callback indirectly calls xwl_present_reset_timer
   -> xwl_present_free_timer
2. xwl_present_timer_callback then returns a non-0 value, so DoTimer
   calls TimerSet with the old xwl_present_window->frame_timer pointer
   which was freed in step 1 => use after free

Calling xwl_present_reset_timer explicitly passes NULL to TimerSet if
step 1 freed xwl_present_window->frame_timer, and it will allocate a new
one.

(cherry picked from commit 5e8b9a3a)