drm/omap: dsi: move panel refresh function to host

This moves the panel refresh/update function from the panel
driver into the DSI host driver to prepare for common drm_panel
support.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-30-tomi.valkeinen@ti.com