ARM: dts: msm: reduce dsc count to 2 for DP

DP driver currently uses this value for number of DSCs available for
external displays without allowing for DSI concurrency. This causes
mode validation failures if DSI also needs to use DSCs. Limiting the
DSC count to 2 for external display to allow for concurrencies.

Change-Id: If760c609c1f304af77f825e211343277ff8d3fe2