msm: mdss: fix boot-up issues with splash thread enabled

Starting with commit f01f7cd13954e86172a5745f5838487b0d2400e5 (delay
iommu mapping of buffers until commit), if continuous splash and splash
thread both are enabled, iommu attach is skipped if handoff is pending.
But if iommu is not attached then iova is treated as physical address and
MDP hw fetches memory from incorrect buffer resulting in corruption on
screen. Fix this by attaching iommu based on its ref count and not by
handoff pending flag. Also if splash thread is disabled then do not allow
iommu attach if it is an empty commit.

Change-Id: I363b87256c8faa6a2ab51e25f3487bc04f91d0dc
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>