drm/atomic: Call fake_vblank() from the generic commit_tail() helpers

Now that we have a way to fake VBLANK events when requested by the CRTC
hook it up to the generic commit_tail() helpers.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703075022.15138-7-boris.brezillon@bootlin.com