glsl: Simplify built-in generator functions for min3/max3/mid3.

glsl: Simplify built-in generator functions for min3/max3/mid3.

The type of all three parameters are identical, so we don't need to
specify it three times.  The predicate is always identical too, so we
don't need to make it a parameter, either.

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