composer: 2.4: vts: Add missing COMPOSER_OVERLAY allocation flag

Without this flag, the composer will fail to PRESENT the buffer on
the systems that do not have IOMMU support and can scanout contiguous-
only buffers.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
Change-Id: I566534a2a1fb602c0ce69ef861805d4fb03d7c47