gpu: ion: Map only the vma size given

When mapping carveout buffers into userspace, only map
the size of the vma given, not the full size of the buffer
since clients may map less than the buffer size.

Change-Id: I1b391b23219455e879a900d947c728db3c8c4a76
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>