drm/bridge: ti-sn65dsi83: Do not cache dsi_lanes and host twice

The DSI lane count can be accessed via the dsi device pointer, make use
of that. The DSI host pointer is only used in sn65dsi83_host_attach(),
move the code around so that the host does not have to be cached in the
driver private data. This simplifies the code further. No functional
change.

This has the added bonus that lt9211, tc358767, sn65dsi83 now use very
similar *_mipi_dsi_host_attach() which is ripe for deduplication.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220612102918.13874-1-marex@denx.de