drm/fence: add fence to drm_pending_event

Now a drm_pending_event can either send a real drm_event or signal a
fence, or both. It allow us to signal via fences when the buffer is
displayed on the screen. Which in turn means that the previous buffer
is not in use anymore and can be freed or sent back to another driver
for processing.

v2: Comments from Daniel Vetter
	- call fence_signal in drm_send_event_locked()
	- remove unneeded !e->event check

v3: Remove drm_pending_event->destroy to fix a leak when e->file_priv
is not set.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk> (v2)
[danvet: fix one e->destroy in arcpgu due to rebasing.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464818821-5736-13-git-send-email-daniel.vetter@ffwll.ch