compositor-drm: Fix inconsistency in finish frame timestamps

The page flip event timestamps comes from the monotonic clock, while
idle_repaint() gets the time with gettimeofday(). That leads to
inconsistent timestamps on the frame callbacks.

Fix this by making the drm backend page flip to the current buffer and
call weston_output_finish_frame() with the page flip event timestamp,
instead of using gettimeofday().