drm: omapdrm: dsi: Don't pass channel to dispc init/uninit functions

The dsi_display_init_dispc() and dsi_display_uninit_dispc() functions
take a channel argument that is reduntant as it is always identical to
the dsi->output.dispc_channel. Remove the argument and use the field
directly in the functions to avoid misuse.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>