When present_wnmd_clear_window_flip is done, present_destroy_window
frees struct present_window_priv, and the events in the flip queue
become unreachable. So if we don't free them first, they're leaked.

Also drop the call to present_wnmd_set_abort_flip, which just sets a
flag in struct present_window_priv and thus can't have any observable
effect after present_destroy_window.

Closes: xorg/xserver#1042


Reviewed-by: Dave Airlie <airlied@redhat.com>