drm/rockchip: lvds: move pclk preparation in with clk_get

The LVDS block needs a separate pclk only on some socs, so currently
requests and prepares it in the soc-specific probe function, but common
code is required to unprepare it in the error path or on driver remove.

While this works because clk_unprepare just does nothing if clk is NULL,
this mismatch of who is responsible still is not very nice.
The clock-framework already has a helper for clk-get-and-prepare even
with devres support in devm_clk_get_prepared().

This will get and prepare the clock and also unprepare it on driver
removal, saving the driver from having to handle it "manually".

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner@cherry.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20250304124418.111061-2-heiko@sntech.de