drm/atomic: Remove WARN_ON for invalid plane configuration.

Userspace can set a FB_ID on a plane without setting CRTC_ID, which
will fail with -EINVAL, but the kernel shouldn't warn about that.

Same for !FB_ID and CRTC_ID being set.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180130102704.28016-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Harry Wentland <harry.wentland@amd.com>

drivers/gpu/drm/drm_atomic.c

@@ -863,10 +863,10 @@ static int drm_atomic_plane_check(struct drm_plane *plane,
 	int ret;
 
 	/* either *both* CRTC and FB must be set, or neither */
-	if (WARN_ON(state->crtc && !state->fb)) {
+	if (state->crtc && !state->fb) {
 		DRM_DEBUG_ATOMIC("CRTC set but no FB\n");
 		return -EINVAL;
-	} else if (WARN_ON(state->fb && !state->crtc)) {
+	} else if (state->fb && !state->crtc) {
 		DRM_DEBUG_ATOMIC("FB set but no CRTC\n");
 		return -EINVAL;
 	}