msm: mhl: sii8334: Fix null pointer dereference at power off

Check for null pointer before disabling the regulators.
During cable connect/disconnect and suspend/resume check for
previous power state of mhl to avoid enabling/disabling gpio's
and regulators twice.

Change-Id: I44dc51122da4630974147b829236cb39c1ad54b0
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>