drm/panel: ilitek-ili9881c: move display_on/_off dcs calls to (un-)prepare

drm/panel: ilitek-ili9881c: move display_on/_off dcs calls to (un-)prepare

At least for panel-bridges, the atomic_enable call is defined as being
called right after the preceding element in the display pipe is enabled.

It is also stated that "The bridge can assume that the display pipe (i.e.
clocks and timing signals) feeding it is running when this callback is
called"

This means the DSI controller driving this display would have already
switched over to video-mode from command mode and thus dcs functions
should not be called anymore at this point.

This caused a non-working display for me, when trying to enable
the rk3576 dsi controller using a display using this controller.

Therefore move the display_on/off calls the more appropriate
prepare/unprepare callbacks.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250707164906.1445288-3-heiko@sntech.de