softpipe: Add support for ARB_derivative_control
Enables and passes piglits:
spec/ARB_drivative_control/ dfdx-coarse dfdx-dfdy dfdx-fine dfdy-coarse dfdy-fine
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Enables and passes piglits:
spec/ARB_drivative_control/ dfdx-coarse dfdx-dfdy dfdx-fine dfdy-coarse dfdy-fine