OMAPDSS: DSI: 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 DSI output, check whether the output entity connected to
display is not NULL.

In dsi_init_display(), the display won't be connected to the DSI output yet,
that happens later in dss_recheck_connections() in the panel driver's probe. Get
the dsidev platform device pointer using the DSI moudle number provided in the
omap_dss_device struct.

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