drm/vc4: Fix NULL pointer dereference in the async update path

vc4_plane_atomic_async_update() calls vc4_plane_atomic_check()
which in turn calls vc4_plane_setup_clipping_and_scaling(), and since
commit 58a6a36fe8e0 ("drm/vc4: Use
drm_atomic_helper_check_plane_state() to simplify the logic"), this
function accesses plane_state->state which will be NULL when called
from the async update path because we're passing the current plane
state, and plane_state->state has been assigned to NULL in
drm_atomic_helper_swap_state().

Pass the new state instead of the current one (the new state has
->state set to a non-NULL value).

Fixes: 58a6a36fe8e0 ("drm/vc4: Use drm_atomic_helper_check_plane_state() to simplify the logic")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181115105852.9844-1-boris.brezillon@bootlin.com

drivers/gpu/drm/vc4/vc4_plane.c

@@ -807,7 +807,7 @@ void vc4_plane_async_set_fb(struct drm_plane *plane, struct drm_framebuffer *fb)
 static void vc4_plane_atomic_async_update(struct drm_plane *plane,
 					  struct drm_plane_state *state)
 {
-	struct vc4_plane_state *vc4_state = to_vc4_plane_state(plane->state);
+	struct vc4_plane_state *vc4_state, *new_vc4_state;
 
 	if (plane->state->fb != state->fb) {
 		vc4_plane_async_set_fb(plane, state->fb);
@@ -828,7 +828,18 @@ static void vc4_plane_atomic_async_update(struct drm_plane *plane,
 	plane->state->src_y = state->src_y;
 
 	/* Update the display list based on the new crtc_x/y. */
-	vc4_plane_atomic_check(plane, plane->state);
+	vc4_plane_atomic_check(plane, state);
+
+	new_vc4_state = to_vc4_plane_state(state);
+	vc4_state = to_vc4_plane_state(plane->state);
+
+	/* Update the current vc4_state pos0, pos2 and ptr0 dlist entries. */
+	vc4_state->dlist[vc4_state->pos0_offset] =
+		new_vc4_state->dlist[vc4_state->pos0_offset];
+	vc4_state->dlist[vc4_state->pos2_offset] =
+		new_vc4_state->dlist[vc4_state->pos2_offset];
+	vc4_state->dlist[vc4_state->ptr0_offset] =
+		new_vc4_state->dlist[vc4_state->ptr0_offset];
 
 	/* Note that we can't just call vc4_plane_write_dlist()
 	 * because that would smash the context data that the HVS is