drm/bridge: analogic_dp: drop panel_lock

The analogix_dp_prepare_panel() function is called from bridge's
atomic_pre_enable() and atomic_post_disable() callbacks, which can not
happen simultaneously. Drop the useless mutex.

Tested-by: Damon Ding <damon.ding@rock-chips.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250401-panel-return-void-v1-4-93e1be33dc8d@oss.qualcomm.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>