drm/rcar-du: dsi: Fix 1/2/3 lane support

drm/rcar-du: dsi: Fix 1/2/3 lane support

Remove fixed PPI lane count setup. The R-Car DSI host is capable
of operating in 1..4 DSI lane mode. Remove the hard-coded 4-lane
configuration from PPI register settings and instead configure
the PPI lane count according to lane count information already
obtained by this driver instance.

Configure TXSETR register to match PPI lane count. The R-Car V4H
Reference Manual R19UH0186EJ0121 Rev.1.21 section 67.2.2.3 Tx Set
Register (TXSETR), field LANECNT description indicates that the
TXSETR register LANECNT bitfield lane count must be configured
such, that it matches lane count configuration in PPISETR register
DLEN bitfield. Make sure the LANECNT and DLEN bitfields are
configured to match.

Fixes: 155358310f01 ("drm: rcar-du: Add R-Car DSI driver")
Cc: stable@vger.kernel.org
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20250813210840.97621-1-marek.vasut+renesas@mailbox.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>