msm: mdss: refactor pipe type checks

There are multiple places in code where pipe type checks are being
done based on the pipe number. With the ids not being sequential this
can be tricky, so add some helper functions to correctly identify pipe
types and use these instead.

CRs-Fixed: 987777
Change-Id: Ie3e5bdd905527dc1e572ca91399b4686d21ae266
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>