drm/msm/dp: fix check for sink support for HDCP 2.x

Current implementation only checks if the sink is HDCP capable.
In addition, it is required that the source also checks for the
HDCP version supported by the sink prior to initiating HDCP 2.x
authentication sequence.

CRs-Fixed: 2062951
Change-Id: Ieed610a799c17e863ca15332a0c36733643fd394
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>