drm: silence variable 'conn' set but not used

The "struct drm_connector" iteration cursor from
"for_each_new_connector_in_state" is never used in atomic_remove_fb()
which generates a compilation warning,

drivers/gpu/drm/drm_framebuffer.c: In function 'atomic_remove_fb':
drivers/gpu/drm/drm_framebuffer.c:838:24: warning: variable 'conn' set
but not used [-Wunused-but-set-variable]

Silence it by marking "conn" __maybe_unused.
Signed-off-by: NQian Cai <cai@lca.pw>
Signed-off-by: NSean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1563822886-13570-1-git-send-email-cai@lca.pw

drivers/gpu/drm/drm_framebuffer.c

@@ -835,7 +835,7 @@ static int atomic_remove_fb(struct drm_framebuffer *fb)
 	struct drm_device *dev = fb->dev;
 	struct drm_atomic_state *state;
 	struct drm_plane *plane;
-	struct drm_connector *conn;
+	struct drm_connector *conn __maybe_unused;
 	struct drm_connector_state *conn_state;
 	int i, ret;
 	unsigned plane_mask;