drm/omap: dsi: Fix OF platform depopulate

Commit edb715df ("drm/omap: dss: dsi: Move initialization code from
bind to probe") moved the of_platform_populate() call from dsi_bind() to
dsi_probe(), but failed to move the corresponding
of_platform_depopulate() from dsi_unbind() to dsi_remove(). This results
in OF child devices being potentially removed multiple times. Fix it by
placing the of_platform_depopulate() call where it belongs.

Fixes: edb715df ("drm/omap: dss: dsi: Move initialization code from bind to probe")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111035120.20668-4-laurent.pinchart@ideasonboard.com