drm/bridge: analogix_dp: drop extra calls to analogix_dp_prepare_panel()

The analogix_dp_prepare_panel() returns immediately if there is no
attached panel. Drop several calls to this function which are performed
when dp->plat_data->panel is NULL.

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-v1-1-93e1be33dc8d@oss.qualcomm.com


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