drm/msm/dp: avoid parsing eDP specific configuration from DPCD

Do not parse eDP configuration capability from the sink's DPCD
as the current implementation only supports DP sink.

CRs-Fixed: 2337670
Change-Id: Iea0d21d4110a89303d7e03f085948685ad4c826d
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>