drm/msm/dp: specify return code for DP HDCP 2.2 init with HDCP disabled

Ensure that all code paths in sde_dp_hdcp2p2_init return a valid
value.  When HDCP is disabled, return a NULL pointer to signal the
error to the caller.

Change-Id: Idd0fe626022d48cb2ac2024a7b5645dc6600e883
Signed-off-by: Steve Cohen <cohens@codeaurora.org>