msm: mdss: Remove 1:1 mixer to DSPP assumption in pp mixer setup

Previously, the PP driver assumed that there would be a DSPP for every
INTF mixer. However, this assumption does not hold true for all targets.
This change fixes the errors caused by assuming the 1:1 ratio and allows
all layer mixers to be programmed based on CTL configuration.

Change-Id: I15d03921c9b91212717417296e3a157e97b5113a
Signed-off-by: Benet Clark <benetc@codeaurora.org>