drm: omapdrm: dsi: Store the struct device pointer in struct dsi_data

The dsi_data structure stores a pointer to a struct platform_device. The
driver only uses the dev member of the platform device structure. Store
the struct device pointer instead and use it directly.

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