disp: msm: sde: reduce complexity in sde_sspp_parse_dt

Reduce the cyclomatic complexity for this function by splitting
the work in to helpers and using the new sde_dt_props method of
device node parsing.

Change-Id: Id4a41225bd78f06ee353a636d17330ba41daf1ff
Signed-off-by: Steve Cohen <cohens@codeaurora.org>