msm: mdss: hdcp2p2: check for valid keys before authentication

HDCP authentication requires valid keys on the source device.
If keys are not present, authentication can never be successful.
Check if the device is provisioned with hdcp keys, if so, proceed
with the hdcp authentication on HDMI/DisplayPort cable connection
otherwise avoid authentication as it may result in unnecessary
re-authentication loop.

Change-Id: I391ee35fa20cfade89773ecb565b220cc6249b8d
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>