drm/panel: himax-hx8394: set prepare_prev_first

drm/panel: himax-hx8394: set prepare_prev_first

Sending DSI commands from the prepare() callback requires DSI link to be
up at that point. For DSI hosts is guaranteed only if the panel driver
sets the .prepare_prev_first flag. Set it to let these panels work with
the DSI hosts which don't power on the link in their .mode_set callback.

Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patch.msgid.link/20260413-waveshare-dsi-touch-v3-8-3aeb53022c32@oss.qualcomm.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>