drm/vmwgfx: Return error code from vmw_execbuf_copy_fence_user

drm/vmwgfx: Return error code from vmw_execbuf_copy_fence_user

The function was unconditionally returning 0, and a caller would have to
rely on the returned fence pointer being NULL to detect errors. However,
the function vmw_execbuf_copy_fence_user() would expect a non-zero error
code in that case and would BUG otherwise.

So make sure we return a proper non-zero error code if the fence pointer
returned is NULL.

Cc: <stable@vger.kernel.org>
Fixes: ae2a104058e2: ("vmwgfx: Implement fence objects")
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>