drm/panel: himax-hx8394: Drop prepare/unprepare tracking

Drop the panel specific prepare/unprepare logic. This is now tracked
by the DRM stack [1].

[1] commit d2aacaf0 ("drm/panel: Check for already prepared/enabled in
drm_panel")

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20231204185719.569021-2-macroalpha82@gmail.com


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231204185719.569021-2-macroalpha82@gmail.com