msm: mdss: send DCS commands in LP mode by default

Recent commit 391a24fe sets
default power mode for DCS commands to HS, because of which
on nt35590_720p video mode and command mode panel suspend resume
fails. So, set the default power mode to LP as it was earlier.

Change-Id: Ifa5090ddc92cc953acc61024d0bdc3d6e93cbb10
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>