msm: mdss: hdmi: protect hpd power from multi-thread access

HPD (Hot Plug Detect) can be powered on or off by down stream
device like MHL or slimport driver based on cable connection
triggered by user. In response to that, HDMI transmitter power
on and off is triggered in a different thread. There can be
a race condition with transmitter power on/off and HPD power
on and off with fast plug in/out. Protect these functionalities
using mutex to avoid such race conditions and keep the HDMI state
machine in valid state.

Change-Id: I7fa8de10b96ca3cf674a0b7cf83b0f96cc177509
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>