msm: mdss: hdmi: Fix unclocked register access during power off

When mirroring is ongoing, if suspend or user specified hpd off is
processed then current implementation turns off hpd first and then cec.
When hpd is turned off, clocks and regulators to HDMI core are disabled.
So any access to HDMI register space leads to crash. Fix this by ensuring
CEC is disabled before HDMI core is powered off.

Change-Id: I171ca71aed27ce6721678920da0e89399201d76d
CRs-Fixed: 502121
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>