msm: mdss: Unregister the regulator notifier when mdss_mdp probe fails

Currently, when mdss_mdp probe fails after registering for
regulator notifications, the regulator_unregister_notifier
function is not called. Because of this, when the regulator
has some state change now, the notification call fails.

Change-Id: I2a41029aa145f88477701ab75a6e30883d2c4017
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>