OMAPDSS: fix warnings if CONFIG_PM_RUNTIME=n

If runtime PM is not enabled in the kernel config, pm_runtime_get_sync()
will always return 1 and pm_runtime_put_sync() will always return
-ENOSYS. pm_runtime_get_sync() returning 1 presents no problem to the
driver, but -ENOSYS from pm_runtime_put_sync() causes the driver to
print a warning.

One option would be to ignore errors returned by pm_runtime_put_sync()
totally, as they only say that the call was unable to put the hardware
into suspend mode.

However, I chose to ignore the returned -ENOSYS explicitly, and print a
warning for other errors, as I think we should get notified if the HW
failed to go to suspend properly.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Jassi Brar <jaswinder.singh@linaro.org>
Cc: Grazvydas Ignotas <notasas@gmail.com>