drm/panel: Allow powering on panel follower after panel is enabled

Some touch controllers have to be powered on after the panel's backlight
is enabled. To support these controllers, introduce .panel_enabled() and
.panel_disabling() to panel_follower_funcs and use them to power on the
device after the panel and its backlight are enabled.

Signed-off-by: Pin-yen Lin <treapking@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20250818115015.2909525-1-treapking@chromium.org