drm/omap: dss: Move and rename omap_dss_(get|put)_device()

The functions operate on any omap_dss_device, move them from display.c
to base.c. While at it rename them to match the naming of the other
functions operating on struct omap_dss_device.

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>