Move deferred vblank events to separate drm_vblank_deferred list

It was still possible for nested xorg_list_for_each_entry_safe loops
to occur over the drm_vblank_signalled list, which could mess up that
list. Moving deferred events to a separate list allows processing the
drm_vblank_signalled list without xorg_list_for_each_entry_safe.

v2:
* Refactor drm_handle_vblank_signalled helper function, less code
  duplication => better readability (Alex Deucher)

Bugzilla: https://bugs.freedesktop.org/108600


Acked-by: Alex Deucher <alexander.deucher@amd.com>