drm/rockchip: dsi: add own additional pclk handling

In a followup patch, we'll need to access the pclk ourself to enable some
functionality, so get and store it in the rockchip dw-dsi variant as well.

Clocks are refcounted, so possible cascading enablements are
no problem.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Tested-by: Sebastian Fricke <sebastian.fricke@posteo.net>
Acked-by: Helen Koike <helen.koike@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210210111020.2476369-2-heiko@sntech.de