dma-mapping-fast: Clean up common code

The current derivation of pfns/page structures has a fair
amount of complexity in it, and can be simplified. For example,
there need not be completely separate cases for pages that are
all remapped into the vmalloc space, but were allocated
differently (i.e. atomic allocations vs contiguous allocations).
So, simplify the handling for those, and leverage iommu_dma_mmap()
and iommu_dma_get_sgtable(), as there is nothing fastmap-specific
with the way pages are mapped into userspace and how we construct
an sg-table respectively.

Change-Id: Ia5725a93ee84f2c5e7ccc1d6e62f828338e83bd7
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>