drm/vc4: Fix potential null pointer read when disabling vblank

vc4_disable_vblank assumed that vc4_encoder was always assigned,
which isn't guaranteed.

If it isn't assigned then disable the interrupt anyway as it's
not connected.

#6146

Fixes: 63c0bcc ("drm/vc4: Add option to call from crtc to encoder on vblank")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

drivers/gpu/drm/vc4/vc4_crtc.c

@@ -834,7 +834,7 @@ static void vc4_disable_vblank(struct drm_crtc *crtc)
 	if (!drm_dev_enter(dev, &idx))
 		return;
 
-	if (vc4_encoder->type != VC4_ENCODER_TYPE_DSI0)
+	if (!vc4_encoder || vc4_encoder->type != VC4_ENCODER_TYPE_DSI0)
 		CRTC_WRITE(PV_INTEN, 0);
 
 	drm_dev_exit(idx);