msm: mdss: Fix the IGC read/write on DSPPs

For msm8974 and earlier targets, there are only 3 DSPP pipes, so
we only need three bits for the DSPP_MASK. However, apq8084 has
four DSPP pipes, hence we need to modify the IGC read/write functions
to correctly configure IGC on all the DSPPs available on targets.

Change-Id: Ief6d27fbe52235919a411189f3a1c8bbef08cb45
Signed-off-by: Ping Li <quicpingli@codeaurora.org>