msm: kgsl: Validate the meta data pointer in kgsl_destroy_ion()

If kgsl_setup_ion() fails for whatever reason entry->priv_data may
either be NULL or pointing to freed memory. In either case, this
is a disaster when kgsl_destroy_ion() tries to use the unvalidated
pointer.  Check for NULL in kgsl_destroy_ion() and make sure that
the entry->priv_data pointer is cleared on error in kgsl_setup_ion.

Change-Id: Ic0dedbad6a13e2d8e15982144102517bc8f4e2e8
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>