ARM: dts: msm: enable idle power collapse on msm8996

On command mode panels, when there are no new frame updates on the
screen, MDSS gdsc can be turned off to save power. This change
enables idle power collapse feature on MSM8996.

Change-Id: I2f31733c548ea70fd7ce7be29d2c3de912fb1e10
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>