drm/panel: novatek-nt36672a: Stop tracking prepared

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.

Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Benni Steini <bennisteinir@gmail.com>
Cc: Marijn Suijten <marijn.suijten@somainline.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Tested-by: Joel Selvaraj <jo@jsfamily.in>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240503143327.RFT.v2.18.I13a06b9e6f5920659b1e5d12543b3cd9066383b8@changeid