mesa: Returns correct error values from gl(Get)SamplerParameter*() on GL-ES 3.0+

mesa: Returns correct error values from gl(Get)SamplerParameter*() on GL-ES 3.0+

'3.8.2 Sampler Objects' section of the GL-ES 3.0 specification states:

    "An INVALID_OPERATION error is generated if sampler is not the name
    of a sampler object previously returned from a call to GenSamplers."

In desktop GL, an GL_INVALID_VALUE is returned instead.

Fixes 6 dEQP failing tests:
* dEQP-GLES3.functional.negative_api.shader.get_sampler_parameteriv
* dEQP-GLES3.functional.negative_api.shader.get_sampler_parameterfv
* dEQP-GLES3.functional.negative_api.shader.sampler_parameteri
* dEQP-GLES3.functional.negative_api.shader.sampler_parameteriv
* dEQP-GLES3.functional.negative_api.shader.sampler_parameterf
* dEQP-GLES3.functional.negative_api.shader.sampler_parameterfv

Reviewed-by: Matt Turner <mattst88@gmail.com>