drm/omap: dsi: lp/hs switching support for transfer()

Integrate low-power / high-speed bus switching into transfer
function and drop the omapdrm specific enable_hs() callback.

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/20201215104657.802264-24-tomi.valkeinen@ti.com