msm: mdss: do not enable the iommu until splash is done

On 8909 backlight is set to zero before stopping dma
in the first commit after continuous splash. But for
the panels which use dcs bakclight commands this will
result in iommu faults as DSI will be attaching iommu
to send the backlight commands. Skip iommu attach in
all cases when continuous splash is still on.

Change-Id: I015a034edda87b83c4ef21a4d47cfd1547d1f1c4
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>