drm/msm/dsi-staging: add support for display selection from command-line

This change adds support to retrieve display nodes
from kernel command-line arguments. These nodes
can be used to choose active displays.

Change-Id: I1b2b64416cd445495de67f9c8eadc7ad82877bd3
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>