drm/omap: add OMAP5 DSIPHY lane-enable support

We are missing OMAP5 DSIPHY lane-enable support, which has prevented
OMAP5 DSI working in mainline. This patch adds the lane-enable similarly
to the recently added OMAP4 version.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>