drm/omap: dsi: add generic transfer function

This prepares the driver for becoming a mipi_dsi_host implementation,
which provides a generic transfer function instead of all kind of
different read/write functions. The implementation will become more
elegant after unexporting the specific functions in the following
patches.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-6-tomi.valkeinen@ti.com