xwayland: Clean up pending eglstream on pixmap destroy

EGLStream implementation in Xwayland keeps a list of pending streams for
a window.

If the windows's pixmap is destroyed while there is a pending stream,
the pending stream will point to freed memory once the callback is

Make sure to cancel the pending stream if there's one when the pixmap is

 * Use xorg_list_for_each_entry() instead of the safe variant (Michel
   Dänzer <mdaenzer@redhat.com>)

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Tested-by: Karol Szuster <karolsz9898@gmail.com>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Closes xorg/xserver#1096
4 jobs for del_pending_stream in 16 minutes and 44 seconds (queued for 19 seconds)