include/drm/drm_atomic: Make use of 'new_crtc_state'

In the macro for_each_oldnew_crtc_in_state() 'crtc_state' is provided
as a container for state->crtcs[i].new_state, but is not utilised in
some use-cases, so we fake-use it instead.

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/imx/ipuv3-plane.c: In function ‘ipu_planes_assign_pre’:
 drivers/gpu/drm/imx/ipuv3-plane.c:746:42: warning: variable ‘crtc_state’ set but not used [-Wunused-but-set-variable]

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-43-lee.jones@linaro.org