drm: rcar-du: lvds: Rename pclk enable/disable functions

The DU driver uses the rcar_lvds_clk_enable() and
rcar_lvds_clk_disable() functions enable or disable the pixel clock
generated by the LVDS encoder, as it requires that clock for proper DU
operation. Rename the functions by replacing "clk" with "pclk" to make
it clearer that they related to the pixel clock.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>