glsl: don't generate extra errors in ValidateProgramPipeline

glsl: don't generate extra errors in ValidateProgramPipeline

From Section 11.1.3.11 (Validation) of the GLES 3.1 spec:

   "An INVALID_OPERATION error is generated by any command that trans-
   fers vertices to the GL or launches compute work if the current set
   of active program objects cannot be executed, for reasons including:"

It then goes on to list the rules we validate in the
_mesa_validate_program_pipeline() function.

For ValidateProgramPipeline the only mention of generating an error is:

   "An INVALID_OPERATION error is generated if pipeline is not a name re-
   turned from a previous call to GenProgramPipelines or if such a name has
   since been deleted by DeleteProgramPipelines,"

Which we handle separately.

This fixes:
ES31-CTS.sepshaderobjs.PipelineApi

No regressions on the eEQP 3.1 tests.

Cc: Gregory Hainaut <gregory.hainaut@gmail.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>