drm/panel: st7701: Add prepare_prev_first flag to drm_panel

drm/panel: st7701: Add prepare_prev_first flag to drm_panel

The DSI host must be enabled for the panel to be initialized in
prepare(). Set the prepare_prev_first flag to guarantee this.
This fixes the panel operation on NXP i.MX8MP SoC / Samsung DSIM
DSI host.

Fixes: 849b2e3ff969 ("drm/panel: Add Sitronix ST7701 panel driver")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Link: https://lore.kernel.org/r/20241124224812.150263-1-marex@denx.de
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241124224812.150263-1-marex@denx.de