compositor-drm: Avoid output_destroy happened before page_flip event

compositor-drm: Avoid output_destroy happened before page_flip event

Currently there is no guarentee that output remove event always happend after
page_flip event on the same output. So if the following situation occur:

  first: unplug a output
  second: output remove event arrive, output_destrory called adn free output
  third: page_flip event arrive on the destroyed output

the segment fault will happpen in page_flip_handler().

This patch add a variable drm_compositor->destroy_pending, if page flip
event is pending when output remove event arrive, output_destroy will be
delayed until page flip finished.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>