drm: Use state helper instead of plane state pointer in atomic_check

Many drivers reference the plane->state pointer in order to get the
current plane state in their atomic_check hook, which would be the old
plane state in the global atomic state since _swap_state hasn't happened
when atomic_check is run.

Use the drm_atomic_get_old_plane_state helper to get that state to make
it more obvious.

This was made using the coccinelle script below:

@ plane_atomic_func @
identifier helpers;
identifier func;
@@

static struct drm_plane_helper_funcs helpers = {
	...,
	.atomic_check = func,
	...,
};

@ replaces_old_state @
identifier plane_atomic_func.func;
identifier plane, state, plane_state;
@@

 func(struct drm_plane *plane, struct drm_atomic_state *state) {
 	...
-	struct drm_plane_state *plane_state = plane->state;
+	struct drm_plane_state *plane_state = drm_atomic_get_old_plane_state(state, plane);
 	...
 }

@@
identifier plane_atomic_func.func;
identifier plane, state, plane_state;
@@

 func(struct drm_plane *plane, struct drm_atomic_state *state) {
 	struct drm_plane_state *plane_state = drm_atomic_get_old_plane_state(state, plane);
 	<...
-	plane->state
+	plane_state
 	...>
 }

@ adds_old_state @
identifier plane_atomic_func.func;
identifier plane, state;
@@

 func(struct drm_plane *plane, struct drm_atomic_state *state) {
+	struct drm_plane_state *old_plane_state = drm_atomic_get_old_plane_state(state, plane);
 	<...
-	plane->state
+	old_plane_state
 	...>
 }

@ include depends on adds_old_state || replaces_old_state @
@@

 #include <drm/drm_atomic.h>

@ no_include depends on !include && (adds_old_state || replaces_old_state) @
@@

+ #include <drm/drm_atomic.h>
  #include <drm/...>

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210219120032.260676-6-maxime@cerno.tech