pwm: mtk-disp: Adjust the clocks to avoid them mismatch

[ Upstream commit d7a4e582 ]

The clks "main" and "mm" are prepared in .probe() (and unprepared in
.remove()). This results in the clocks being on during suspend which
results in unnecessarily increased power consumption.

Remove the clock operations from .probe() and .remove(). Add the
clk_prepare_enable() in .enable() and the clk_disable_unprepare() in
.disable().

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
[thierry.reding@gmail.com: squashed in fixup patch]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Stable-dep-of: 36dd7f53 ("pwm: mtk-disp: Disable shadow registers before setting backlight values")
Signed-off-by: Sasha Levin <sashal@kernel.org>