nir: Silence unused parameter warnings in nir_constant_expression.c

nir: Silence unused parameter warnings in nir_constant_expression.c

nir/nir_constant_expressions.c:290:25: warning: unused parameter 'num_components' [-Wunused-parameter]
 evaluate_ball3(unsigned num_components, nir_const_value *_src)
                         ^
nir/nir_constant_expressions.c: In function 'evaluate_fddx':
nir/nir_constant_expressions.c:1282:57: warning: unused parameter '_src' [-Wunused-parameter]
 evaluate_fddx(unsigned num_components, nir_const_value *_src)
                                                         ^

v2: Unconditionally mark the parameters as MAYBE_UNUSED instead of
conditionally adding (void) casts to keep the generator simple.
Suggested by Jason.

Number of total warnings in my build reduced from 1575 to 1485
(reduction of 89).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>