drm/panel: make prepare/enable and disable/unprepare calls return void

Now there are no users of the return value of the drm_panel_prepare(),
drm_panel_unprepare(), drm_panel_enable() and drm_panel_disable() calls.
Usually these calls are performed from the atomic callbacks, where it is
impossible to return an error. Stop returning error codes and return
void instead.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250401-panel-return-void-v1-7-93e1be33dc8d@oss.qualcomm.com


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