drm/imx: ignore plane updates on disabled crtcs

This patch fixes backtraces like the following when sending SIGKILL to a
process with a currently pending plane update:

    [drm:ipu_plane_atomic_check] CRTC should be enabled
    [drm:drm_framebuffer_remove] *ERROR* failed to commit
    ------------[ cut here ]------------
    WARNING: CPU: 3 PID: 63 at drivers/gpu/drm/drm_framebuffer.c:926 drm_framebuffer_remove+0x47c/0x498
    atomic remove_fb failed with -22
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

drivers/gpu/drm/imx/ipuv3-plane.c

@@ -370,9 +370,9 @@ static int ipu_plane_atomic_check(struct drm_plane *plane,
 	if (ret)
 		return ret;
 
-	/* CRTC should be enabled */
+	/* nothing to check when disabling or disabled */
 	if (!crtc_state->enable)
-		return -EINVAL;
+		return 0;
 
 	switch (plane->type) {
 	case DRM_PLANE_TYPE_PRIMARY: