drm/msm/dp: avoid sink message processing when hdcp is off

Skip processing the sink message in sde_hdcp_2x_main thread when hdcp
TA is unloaded. If there is any error generated during the
authentication sequence, then the hdcp authentication is aborted and
the source will restart authentication procedure. When authentication
is aborted, the qseecom handle is lost and memory it referenced
becomes protected. When a subsequent command execution in the
sde_hdcp_2x_main thread tries to dereference that particular memory
from the qseecom handle, there is a page fault crash observed.

This change will skip accessing the memory referenced by the qseecom
handle when hdcp authentication session is tearing down.

Change-Id: I71ed44042897f17cc8817a1b1c54a43e42cf5f73
Signed-off-by: Rajat Gupta <rajatgu@codeaurora.org>