As DMA addr pool for display unsecure context bank includes
cont splah region it can be used by first commit. If we try
to unmap the cont splash region after splash handoff while
it is being used, SMMU faults are seen. This change leaves
a hole at the cont splash in DMA address pool to avoid using
this region for unsec_fbs.
Change-Id: Ifc7e1875d5def8bde0b94589b37e78007a0312b6