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.

For the s6e63m0 panel driver, this actually fixes a subtle/minor error
handling bug in s6e63m0_prepare(). In one error case s6e63m0_prepare()
called s6e63m0_unprepare() directly if there was an error. This call
to s6e63m0_unprepare() would have been a no-op since ctx->prepared
wasn't set yet.

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.2.Iabafd062e70f6b6b554cf23eeb75f57a80f7e985@changeid