drm/armada: redo locking and atomics for armada_drm_crtc_complete_frame_work()

We can do better with armada_drm_crtc_complete_frame_work() - we can
avoid taking the event lock unless a call to drm_send_vblank_event()
is required, and using cmpxchg() and xchg(), we can eliminate the
locking around dcrtc->frame_work entirely.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>