OMAPDSS: VENC: Replace dssdev->manager with dssdev->output->manager references

With addition of output entities, a device connects to an output, and an output
connects to overlay manager. Replace the dssdev->manager references with
dssdev->output->manager to access the manager correctly.

When enabling the VENC output, check whether the output entity connected to
display is not NULL.

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