msm: mdss: fix ulps during suspend feature logic

In current implementation DSI interface state flag is
used to enter into ulps mode during suspend if ulps in
suspend feature is enabled. But this DSI interface state
flag will be still set by the time call comes to ulps config
function, since we need DSI controller to be in active state
to enter into ulps mode. This change puts a proper check of
DSI panel state flag to enter into ulps during suspend, instead
of DSI interface state flag.

Change-Id: I55b6498ded60957dc2a4724140ba8153cb658418
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>