drm/bridge: lt9611: Fix handling of 4k panels

4k requires two dsi pipes, so don't report MODE_OK when only a
single pipe is configured. But rather report MODE_PANEL to
signal that requirements of the panel are not being met.

Reported-by: Peter Collingbourne <pcc@google.com>
Suggested-by: Peter Collingbourne <pcc@google.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Tested-by: John Stultz <john.stultz@linaro.org>