msm: mdss: refactor device tree pipe parsing logic

Current logic is not properly scalable and hard to maintain, and makes
assumption on the order of pipe numbers. Simplify the logic in order
to be more generic and combine common code into a single helper
function which is easier to maintain.

CRs-Fixed: 987777
Change-Id: Ieb2dccee116e155476fca953bf583f0869968b59
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>