drm/msm/dsi-staging: optimize dsi probe

Currently every dsi display is probed. Optimize it by having
a main dsi device tree node and reference all available displays.
Use the command line parameters to identify which display to
select for a given target.

Change-Id: Ic88d431829b177afdc91b2282b630ea0356d886a
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>