msm: kgsl: Don't allow userspace to pass in an invalid size

We currently allow userspace to pass in size zero for our cache
range operations without any checks. Calling dma cache range
operations with zero size is very problematic and results in a
kernel panic.

Change-Id: Ic60d622c06b26e55296aed584e8c87bc9ad3ca37
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>