msm: kgsl: Zero the pointer to the dmabuf sg list before freeing

We borrow the sg table pointer from the dmabuf if it exists instead
of creating our own.  If it isn't zeroed after calling
kgsl_destroy_ion() then kgsl_sharedmem_free() thinks we own it tries
to walk it / free it.  Since the buffer has been previously freed
bad things happen.

Change-Id: Ic0dedbadbf186a47b8533cc758ff0142744aa8b2
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>