OMAPDSS: APPLY: Don't check manager settings if it is disabled

If a manager is disabled, there is no guarantee at any point in time that all
it's parameters are configured. There is always a chance that some more
parameters are yet to be configured by a user of DSS, or by DSS itself.

However, when the manager is enabled, we can be certain that all the parameters
have been configured, as we can't enable a manager with an incomplete
configuration. Therefore, if a manager is disabled, don't check for the validity
of it's parameters or the parameters of the overlays connected to it. Only check
once it is enabled. Add a check in dss_check_settings_low() to achieve the same.

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