drm/omap: remove global dss_device variable

We can simply provide the device to the omapdrm driver
via pdata. omapdss_is_initialized() is no longer required
(even before this patch), since omapdrm device is only
registered after the pointer is initialized.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-39-tomi.valkeinen@ti.com