OMAPDSS: APPLY: Remove omap_dss_device references from dss_ovl_enable/disable

An overlay isn't allowed to be enabled/disabled if it isn't connected to an
omap_dss_device. This requirement isn't needed any more. An overlay can be
enabled/disabled as long as it has an output connected to it. The output may
not be connected to a device, but we can be assured that the connected
manager's output is in use by an output interface.

Signed-off-by: Archit Taneja <archit@ti.com>