kms: Return a negative error code in kms_bo_create()

The function returns returns 0 on success or a negative value in case of an
error, except when given invalid attributes in which case it returns the
positive EINVAL value. Replace that with -EINVAL to allow the caller to detect
errors with a < 0 check.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jakob Bornecrantz <wallbraker@gmail.com>