drm/omap: dss: Get regulators at probe time

Regulators for the DPI, DSI, HDMI, SDI and VENC outputs are all looked
up when connecting the output omap_dss_device. There's no need to delay
regulator handling to that time, get the regulators at probe time.

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>