mesa: fix ES only draw if we have vertex positions

mesa: fix ES only draw if we have vertex positions

This code was separated from the validation code so it could
use used with KHR_no_error paths. The return values were inverted
to reflect the name of the helper, but here the condtion was
mistakenly inverted rather than the return value.

Fixes: 4df2931a87fe (mesa/vbo: move some Draw checks out of validation)
Reported-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>