msm: mdss: indicate splash screen condition explicitly

Earlier, we were using if the iommu is attached condition
to check if it is in the splash screen. In the command
mode panel, as part of xo shutdown, we need to detach the
iommu as well, in the idle screen case. So, it is not a
proper condition to determine if it is in the splash screen.

Change-Id: I1740e381abf2e4a8989cfb2f2f7f4819363ffaa2
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>