mesa: Allows querying GL_SAMPLER_BINDING on GLES3 profile

From GLES3 specification (page 123), "The currently bound sampler may be
queried by calling GetIntegerv with pname set to
SAMPLER_BINDINGGL_SAMPLER_BINDING".

Fixes 4 dEQP tests:
* dEQP-GLES3.functional.state_query.integers.sampler_binding_getboolean
* dEQP-GLES3.functional.state_query.integers.sampler_binding_getinteger
* dEQP-GLES3.functional.state_query.integers.sampler_binding_getinteger64
* dEQP-GLES3.functional.state_query.integers.sampler_binding_getfloat