disp: msm: dp: parse displayport label dt property

Add logic in DP driver to parse "label" dt property.
This info will be useful to configure DP as primary
or secondary.

Change-Id: I708179c5940c82256034bc6c8338b01ee52f2f79
Signed-off-by: Narender Ankam <nankam@codeaurora.org>