drm/panel: Don't store+check prepared/enabled for simple cases

As talked about in commit d2aacaf0 ("drm/panel: Check for already
prepared/enabled in drm_panel"), we want to remove needless code from
panel drivers that was storing and double-checking the
prepared/enabled state. Even if someone was relying on the
double-check before, that double-check is now in the core and not
needed in individual drivers.

This pile of panel drivers appears to be simple to handle. Based on
code inspection they seemed to be using the prepared/enabled state
simply for double-checking that nothing else in the kernel called them
inconsistently. Now that the core drm_panel is doing the double
checking (and warning) it should be very clear that these devices
don't need their own double-check.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230804140605.RFC.1.Ia54954fd2f7645c1b86597494902973f57feeb71@changeid