drm: Add old state pointer to CRTC .enable() helper function

drm: Add old state pointer to CRTC .enable() helper function

The old state is useful for drivers that need to perform operations at
enable time that depend on the transition between the old and new
states.

While at it, rename the operation to .atomic_enable() to be consistent
with .atomic_disable(), as the .enable() operation is used by atomic
helpers only.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com> # for sun4i
Acked-by: Philipp Zabel <p.zabel@pengutronix.de> # for imx-drm and mediatek
Acked-by: Alexey Brodkin <abrodkin@synopsys.com> # for arcpgu
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com> # for atmel-hlcdc
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com> # for hdlcd and mali-dp
Acked-by: Stefan Agner <stefan@agner.ch> # for fsl-dcu
Tested-by: Philippe Cornu <philippe.cornu@st.com> # for stm
Acked-by: Philippe Cornu <philippe.cornu@st.com> # for stm
Acked-by: Vincent Abriou <vincent.abriou@st.com> # for sti
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com> # for vmwgfx
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170630093646.7928-2-laurent.pinchart+renesas@ideasonboard.com