msm: mdss: revisit igc pipe enumeration logic

When setting up IGC, current code makes assumptions on how the DMA
pipes are ordered in enum, but this may not be the case. Refactor code
such that the index is properly obtained by iterating through pipe
list in order.

CRs-Fixed: 987777
Change-Id: I56f9fa0919a7f961cdb6c07f5126fd5e3aa1c678
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>