msm: mdss: add support for continuous splash on 8909

Add changes in mdp3 driver to support new method of
reserving the splash memory in the dt file. Also add
changes for splash to work with DSI 6G. And make sure
that continuous splash is stopped before enabling iommu
by BLIT call as DMA and PPP share the single context
bank on 8909.

Change-Id: I43b3c6f24fd12c40eb97cb91117c44fd69647a36
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>