drm/bridge: analogix_dp: drop unused argument to analogix_dp_prepare_panel()

After previous cleanup all calling sites pass true as is_modeset_prepare
argument to analogix_dp_prepare_panel(). Drop dead code depending on
that argument being false.

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-v1-2-93e1be33dc8d@oss.qualcomm.com


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