msm: mdss: disable ECG feature on 28nm PHY platform

On platform which use 28nm DSI PHY PLL, reconfiguring DSI
clocks as part of Early Clock Gating feature takes significant
time which in turn is causing power regressions. So disable
ECG feature on those platforms.

Change-Id: I41df9dd46138c472606c4479b14a225346816038
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>