drm/msm/hdcp: set hardware key once per session

Set the hardware key only once per session to prevent redundant calls
into the secure layer, and repeatedly notifying the DisplayPort
driver of authentication success.

CRs-Fixed: 2449071
Change-Id: If6db8de27ee61d86308dc3fd4ac48377fbbd24f8
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>