msm: mdss: hdmi: check encryption status only when panel is on

Check the encryption status only when the panel is on so that
there is no potential for un-clocked register access.
Furthermore, protect the read of the panel/hpd status variables
by using the global lock since these variables change state
in different threads.

CRs-FIxed: 1045535
Change-Id: Ifd0edd0deef2d8336a0e867f636bce89bb0b57e1
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>