drm/sde/hdcp: Update sink sync check to be disabled by default

The DP standard leaves ambiguity to if sink synchronization
will be set before auxiliary protocols such as HDCP are to begin.
Update the dp_display HDCP worker  to check sink sync only if
explicitly instructed to via the new debugfs node 'hdcp_wait_sink_sync'

Change-Id: I4f4d0b14096aa3dc498e47de557de14092d37a58
Signed-off-by: Christopher Braga <cbraga@codeaurora.org>