Refactor the validation out from the API and into a separate Validate
function. Also check the sampler parameter first to match dEQP
expectations.

BUG=angleproject:1101
TEST=dEQP-GLES3.functional.negative_api.shader.sampler* (all pass)

Change-Id: I5f4072d9e52d37f741bd4c90f1bffe13371af3f5
Reviewed-on: https://chromium-review.googlesource.com/336162Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>