drm: omapdrm: dss: Don't unnecessarily cast to dev to pdev and back

The dss_unbind() function casts the struct device pointer to a struct
platform_device, only to later use the struct device pointer from
platform_device. Don't cast at all.

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