mesa: remove link validation that should be done elsewhere

mesa: remove link validation that should be done elsewhere

Even if re-linking fails rendering shouldn't fail as the previous
succesfully linked program will still be available. It also shouldn't
be possible to have an unlinked program as part of the current rendering
state.

This fixes a subtest in:
ES31-CTS.sepshaderobjs.StateInteraction

This change should improve performance on CPU limited benchmarks as noted
in commit d6c6b186cf308f.

>From Section 7.3 (Program Objects) of the OpenGL 4.5 spec:

   "If a program object that is active for any shader stage is re-linked
    unsuccessfully, the link status will be set to FALSE, but any existing
    executables and associated state will remain part of the current rendering
    state until a subsequent call to UseProgram, UseProgramStages, or
    BindProgramPipeline removes them from use. If such a program is attached to
    any program pipeline object, the existing executables and associated state
    will remain part of the program pipeline object until a subsequent call to
    UseProgramStages removes them from use. An unsuccessfully linked program may
    not be made part of the current rendering state by UseProgram or added to
    program pipeline objects by UseProgramStages until it is successfully
    re-linked."

   "void UseProgram(uint program);

   ...

   An INVALID_OPERATION error is generated if program has not been linked, or
   was last linked unsuccessfully.  The current rendering state is not modified."

V2: apply the rule to both core and compat.

Cc: Tapani Pälli <tapani.palli@intel.com>
Cc: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>