drm/panel: visionox-rm69299: Make use of prepare_prev_first

drm/panel: visionox-rm69299: Make use of prepare_prev_first

The DSI link must be powered up to let panel driver to talk to the panel
during prepare() callback execution. Set the prepare_prev_first flag to
guarantee this.

Fixes: 9e15123eca79 ("drm/msm/dsi: Stop unconditionally powering up DSI hosts at modeset")
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: David Heidelberg <david@ixit.cz>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patch.msgid.link/20260417-axolotl-display-v2-1-8ce5341e46c2@ixit.cz