drm/omap: dsi: Fix missing of_platform_depopulate()

We're missing a call to of_platform_depopulate() on errors for dsi.
Looks like dss is already doing this.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181106152802.38599-1-tony@atomide.com