msm: mdss: Move iommu attach after overlay start in pan display

IOMMU page faults can happen if iommu is attached in pan display
if continuous splash is enabled as splash fetches from physical
memory. Splash cleanup is performed in overlay start and hence
move iommu attach after splash cleanup is done.

Change-Id: I703c925f964527f33108a3a09ee1ffdc0a3003cd
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>