msm: kgsl: Check for zero length buffers with ION mappings

When ION buffers are mapped into GPU virtual address space,
trigger an error if a buffer of size zero is requested to be mapped.

Change-Id: I59d9dcee0b6eaf38f886a6b5b84b5fb7b16c4c06
Acked-by: Sunil Kumar <sdevalok@qualcomm.com>
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>