Wait for pending scanout update before calling drmmode_crtc_scanout_free

There is a case that when set screen from reverse to normal, the old
scanout damage is freed in modesetting before scanout update handler,
so it causes segment fault issue.

Signed-off-by: Jim Qu <Jim.Qu@amd.com>

[ Michel Dänzer: Only call drmmode_crtc_wait_pending_event before
  drmmode_crtc_scanout_free is actually called, slightly tweak commit
  message ]
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
(Ported from amdgpu commit 9f6a8905611b5b1d8fcd31bebbc9af7ca1355cc3)
Acked-by: Alex Deucher <alexander.deucher@amd.com>