drm: omapdrm: dsi: Pass the dsi_data pointer to internal functions

Internal dsi functions take a pointer to the DSI platform_device and
then cast it to a dsi_data pointer. That's pointless as the caller
already has the dsi_data pointer. Pass it directly instead of the
platform_device pointer.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>