msm: mdss: Avoid disabling regulator again which is disabled

Multiple regulator disable call puts the regulator driver in bad state.
Added regulator status check before disable regulator to avoid multiple
regulator disable call.

Change-Id: I6b60384e16ca5d7bfa2a7cb04d1d1344573bf419
Signed-off-by: Sachin Bhayare <sachin.bhayare@codeaurora.org>