drm: renesas: rz-du: mipi_dsi: Set DSI divider

drm: renesas: rz-du: mipi_dsi: Set DSI divider

Before the MIPI DSI clock source can be configured, the target divide
ratio needs to be set.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Tested-by: Biju Das <biju.das.jz@bp.renesas.com>
Fixes: 5a4326f2e3b1 ("clk: renesas: rzg2l: Remove DSI clock rate restrictions")
Link: https://patch.msgid.link/20260227015216.2721504-1-chris.brandt@renesas.com
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>