drm: rcar-du: lvds: Add API to enable/disable clock output

On the D3 and E3 platforms, the LVDS internal PLL supplies the pixel
clock to the DU. This works automatically for LVDS outputs as the LVDS
encoder is enabled through the bridge API, enabling the internal PLL and
clock output. However, when using the DU DPAD output with the LVDS
outputs turned off, the LVDS PLL needs to be controlled manually. Add an
API to do so, to be called by the DU driver.

The drivers/gpu/drm/rcar-du/ directory has to be treated as obj-y
unconditionally, as the LVDS driver could be built-in while the DU
driver is compiled as a module.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>