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.

The conversion of the rm67191 panel driver follows many of the other
panel drivers but has a few differences that need to be called out.

Like in commit 1e0465eb ("drm/panel: otm8009a: Don't double check
prepared/enabled"), this panel also uses the "prepared" flag to
prevent the backlight functions from running when the panel is powered
off. This is probably not the safest thing to do but the old behavior
was preserved. See the discussion in the otm8009a patch. Because of
this, I've left the driver tracking "prepared" but removed its
tracking of "enabled".

NOTE: as part of this, transition the panel's direct calls to its
disable/unprepare functions in shutdown to call through DRM panel.

Cc: Robert Chiras <robert.chiras@nxp.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20240604172305.v3.21.I20f82e9dd1597a14ae37a64c6b8275add60fbdb1@changeidSigned-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240604172305.v3.21.I20f82e9dd1597a14ae37a64c6b8275add60fbdb1@changeid