drm: rcar-du: dsi: Fix VCLKSET write

rcar_mipi_dsi_startup() writes correct values to VCLKSET, but as it uses
or-operation to add the new values to the current value in the register,
it should first make sure the fields are cleared.

Do this by using rcar_mipi_dsi_write() to write the VCLKSET register
with a variable that has all the unused bits zeroed.

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>