msm: mdss: Add check for enabled regulators to avoid delays

If the regulator is already enabled, driver doesn't need to
wait till post on delay is complete. This change fixes to avoid
delays during regulator enable.

Change-Id: If7c7d1f2895abaa12b94e9f5c78ba61f28b87758
Signed-off-by: Jie Cheng <rockiec@codeaurora.org>
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>