drm/vc4: Use __drm_atomic_helper_crtc_reset

Now also comes with the added benefit of doing a drm_crtc_vblank_off(),
which means vblank state isn't ill-defined and fail-y at driver load
before the first modeset on each crtc.

Reviewed-by: Maxime Ripard <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Cc: Eric Anholt <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vc4/vc4_crtc.c

···
 {
 	if (crtc->state)
 		vc4_crtc_destroy_state(crtc, crtc->state);
-
 	crtc->state = kzalloc(sizeof(struct vc4_crtc_state), GFP_KERNEL);
 	if (crtc->state)
-		crtc->state->crtc = crtc;
+		__drm_atomic_helper_crtc_reset(crtc, crtc->state);
 }
 
 static const struct drm_crtc_funcs vc4_crtc_funcs = {