mesa: return initial value for PROGRAM_SEPARABLE when not linked

mesa: return initial value for PROGRAM_SEPARABLE when not linked

From OpenGL ES 3.1 spec (7.12):

    "Most properties set within program objects are specified not to
    take effect until the next call to LinkProgram or ProgramBinary.
    Some properties further require a successful call to either of
    these commands before taking effect. GetProgramiv returns the
    properties currently in effect for program, which may differ from
    the properties set within program since the most recent call to
    LinkProgram or ProgramBinary, which have not yet taken effect. If
    there has been no such call putting changes to pname into effect,
    initial values are returned."

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Marta Lofstedt <marta.lofstedt@intel.com>