drm/meson: Fix plane atomic check when no crtc for the plane

When no CRTC is associated with the plane, the meson_plane_atomic_check()
call breaks the kernel with an Oops.

Fixes: bbbe775ec5b5 ("drm: Add support for Amlogic Meson Graphic Controller")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>

drivers/gpu/drm/meson/meson_plane.c

@@ -51,6 +51,9 @@ static int meson_plane_atomic_check(struct drm_plane *plane,
 	struct drm_crtc_state *crtc_state;
 	struct drm_rect clip = { 0, };
 
+	if (!state->crtc)
+		return 0;
+
 	crtc_state = drm_atomic_get_crtc_state(state->state, state->crtc);
 	if (IS_ERR(crtc_state))
 		return PTR_ERR(crtc_state);