drm/omap: dsi: Simplify debugfs implementation

The DSI debugfs regs and irqs show handlers received a pointer to the
DSI private data. There's no need to look it up from the list of DSS
outputs. Use the pointer directly, this allows simplifying the
implementation of the handlers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>