gpu: ion: Allow smaller mappings

Clients using ION must be able to mmap a smaller
set of pages than what was allocated from the
IOMMU heap.

Add check that we don't try to map a page passed
the end of the vma.

Change-Id: I1241f952359f6bb1d5bb0deb11cc0f44d94a3404
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>