drm/omap: dsi: simplify read functions

Simplify the read related message handling by using the functionality
provided by CONFIG_DRM_MIPI_DSI.

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/20201215104657.802264-11-tomi.valkeinen@ti.com