drm/omap: dsi: request VC via mipi_dsi_attach

Drop custom request_vc/release_vc callbacks by using the
generic mipi_dsi_attach/mipi_dsi_detach functions.

To use mipi_dsi_attach() we need to fill in the mipi_dsi_device fields,
and some of these fields overlap with the fields in omap_dss_dsi_config.
In later patches the latter will get dropped.

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/20201215104657.802264-15-tomi.valkeinen@ti.com