pwm: mtk-disp: Disable shadow registers before setting backlight values

stable inclusion
from stable-v5.10.180
commit 45e4c00940beecc3a81934d8e4d58ce67d9f88c2
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8FC2O

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=45e4c00940beecc3a81934d8e4d58ce67d9f88c2



--------------------------------

[ Upstream commit 36dd7f53 ]

If shadow registers usage is not desired, disable that before performing
any write to CON0/1 registers in the .apply() callback, otherwise we may
lose clkdiv or period/width updates.

Fixes: cd4b45ac ("pwm: Add MediaTek MT2701 display PWM driver support")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Tested-by: Alexandre Mergnat <amergnat@baylibre.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>