drm/panel: Add prepare_prev_first flag to drm_panel

Mapping to the drm_bridge flag pre_enable_prev_first,
add a new flag prepare_prev_first to drm_panel to allow
the panel driver to request that the upstream bridge should
be pre_enabled before the panel prepare.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20221205173328.1395350-6-dave.stevenson@raspberrypi.com
Signed-off-by: Maxime Ripard <maxime@cerno.tech>