pwm: mtk-disp: Fix the parameters calculated by the enabled flag of disp_pwm

In the original mtk_disp_pwm_get_state() function wrongly uses bit 0 of
CON0 to judge if the PWM is enabled.
However that is indicated by a bit (at a machine dependent position) in
the DISP_PWM_EN register. Fix this accordingly.

Fixes: 3f2b1673

 ("pwm: mtk-disp: Implement atomic API .get_state()")
Signed-off-by: xinlei lee <xinlei.lee@mediatek.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/1666172538-11652-1-git-send-email-xinlei.lee@mediatek.com
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>